Faraday report to Trinity House   16 January 1860

Royal Institution | 16 January 1860.

On Moulded Glass

The specimens of moulded glass received from France are intended for optical use in lighthouses, and are like those that are already applied in the third order light established on the Point du Walde near Calais. Instead of being wrought into shape by machinery, they are pressed into shape in moulds whilst hot. Instead of each rib being separate, many ribs are contained in one piece of glass. As the ribs may be numerous without increased expence, the whole thickness of the glass traversed by the light may be reduced much less than in the case of worked ribs; so that less absorption of light is caused, yet with a glass surface considered sufficiently perfect and at a greatly diminished expence in the manufacture.

Hitherto the tendency has been to raise the optical character of the lighthouse apparatus on every point: to render the glass more clear & colourless; - to diminish the presence of striae and bubbles; & to perfect the form of the glass, and the character of its surface, so as to refract the rays of light properly and send them with as little deviation as possible on their way. I think this endeavour has been in the right direction. High perfection has a moral influence on the keepers and all concerned and helps to sustain the standard of lighthouse duty; - it is highly advantageous with the use of the ordinary oil lamp - and if hereafter, the employment of intense compressed sources of light, as the Electric lamp or the oxy lime light, should be successfully introduced, it will be of the utmost consequence to have the power of applying optical apparatus of diminished size, which can only be done by the use of perfect means & workmanship.

The following is an account of the pieces of glass sent to the Trinity House.

Pieces A/1 and A/2 being put together edge ways form a plano convex lens 20.2 inches in height and 10 inches in width having a focal distance of 11.8 inches. Besides the central lens portion, there are 16 rings either whole or in part round this center, making up the rest of the lens. As to the light it received from a lamp when in place it is equal to 80° in the vertical direction & 45° in the horizontal direction[.]

Pieces B/1 and B/2 being put together edgeways, form a pannel for a fixed light: - its height is 19½ inches - its width 5½ and its focal distance 8.87 inches. The ribs are horizontal and altogether are 32 in number. 95° vertical of light are received by this pannel[.]

Pieces D/1, D/2, D/3 and D/4, placed edge to edge, form a pannel for a fixed light having 19 inches height and a width of 8½ inches. The focal distance is 6 inches. The ribs are of course horizontal. The whole number is 48, there being 8 in each of the middle pieces and 16 in each of the extreme pieces. The vertical angle of light received is 116°[.]

Pieces E/1 and E/2, placed edge to edge, make a pannel for a fixed light, 12 inches high and 6½ wide. The focal distance is 5.25 inches. The ribs are of course horizontal and many of them very fine; there being 70 in the 12 inches of height[.] The vertical angle of light received is 96°.

All these pieces have been cast with pressure in moulds. In many parts the fire surfaces have been left upon them but as these have a certain slight irregularity of form (& perhaps degree of dullness) which tends to disperse light; wherever the surfaces are large enough to permit of it they have been brought under the tool & polished: not for the sake of giving form to the piece as a whole, but of removing these minute irregularities of the surface, & rendering it clear & uniform.

The glass used is flint glass, it being softer when hot than plate glass, & so taking a sharper impression in the moulds. It is excellent in colour i.e it is nearly colourless. Some pieces contain striae rather large in size, but the greater number are very good in that respect. Striae are more likely to occur in flint than in plate glass.

The chief point of comparison (speaking optically) between the pressed glass and the wrought or cut glass is in their power of accurate refraction so as to send the light ray in the required direction; which, excluding colour transparency striae &c, will depend upon the general angular form of the glass, and upon the degree of perfection & truthfulness of its surfaces[.] The refractive power of the moulded glass pieces was very good; and remarkable, considering that they are the result of very early attempts; but they will not compare except at a considerable distance, with well wrought cut glass. The difference is evident when a lamp is placed in the focus of the pieces, & the course of the outgoing rays observed. It is more evident when parallel rays, as those of the sun, are received upon the exterior of the pieces & their collection & concentration on a screen placed in the focus observed;- or when the rays from a distant electric lamp, with its intense but compressed light are employed in like manner, and the image in the focus examined. The pieces have been examined in all these ways but it will be sufficient to state the result with the Electric lamp only.

In order to have a point of comparison a lens of wrought glass was procured consisting of a central plano-convex part, & two concentric rings: it was 12 inches in diameter & had a focal distance of 9.75 inches. The electric lamp placed at the distance of 41 feet gave a very bright & concentrated focal image, not more than 1/8 of an inch in diameter surrounded by a small circle of blue & another of red light due to the difference in refraction of the different rays; these effects showed that the lens was excellent in workmanship. When the same experiment was made with the pieces A/1 and A/2, the image in the focus was much more diffused & much less bright in the centre: - and though it is difficult to define the limit of an illuminated spot which gradually fades from the centre outwards, yet a space of from 2 to 3 inches broad was more or less considerably illuminated at that place & distance when the focal light was most compressed together and what may be considered as the brightest part was half an inch or more broad. The focal distance is given at 12 inches but I think the most compressed usage was probably at 13 inches; but the greatest compression is difficult to judge of through 2 inches or more A/1 A/2 The effects produced when an ordinary lamp was placed in the focus of each of these lenses & the outgoing rays were observed at a distance corresponded with the indication thus obtained[.]

It will be seen that as the focal distance of the wrought lens was 9.75 inches and that of the moulded lens 12 inches the former is in that respect under a greater disadvantage than the latter: - that as the angle of rays received by the former was 63° that number squared and multiplied by 0.7854 shews that 3120 may represent the light falling on it from a central lamp whilst as the vertical angle of the pressed lens was 80° and its horizontal angle 45°, 3600 may represent the light falling on it; so that as regarded external illumination the disadvantage was still, in that respect, against the wrought lens. Again the greatest thickness of the wrought glass was 1½ inches whilst that of the pressed glass was only 6/8 or only half as much. Notwithstanding these points the wrought glass did its work better and gave a much better & brighter image either inwards or outwards than the pressed glass.

The pieces B/1 B/2 are intended to give a fixed light: and with the external electric light the compression of the focal line is to be estimated at that spot of most compression within the pannel in the axial lens of the ray[.] As the pannel is 19½ inches high and has a focal distance of only 9 inches, it might be expected, if of cut glass, to give a wider image in the focus than the cut lens above referred to did. Experimentally it gave a very diffused image being an inch wide vertically & the brightest part 3/8 of an inch wide so far corresponding with the action of pressed lens above referred to, but I think the general effect of this pannel was more correct than that of the lens A/1.2.

The pannel D/1.2.3.4. gave the most compressed focal band at the distance of 5½ inches; the given focus being 6 inches off. The band was pretty well defined at the edges, but broad, being 9/16 of an inch wide. The upper & lower pieces of glass did not appear to be very good they added little to the general effect & when acting without the middle pieces gave a band of diffuse light more than 2 inches wide & having a different final distance to the two middle pieces. The latter were very efficient[.]

The pannel E/1.2. is like the former in character and gives its best focal image at 5½ inches; its given radius being 5.25 inches. The whole band was ½ wide, of which 2/8 in the middle were brightest & the upper & lower portions less bright[.]

When a lamp is placed in the foci of these pannels they give corresponding results in the external illumination at a distance. When this distance was made in succession 41 & 98 feet B gave more light than D and D was better than E.

When the three catadioptric reflecting pieces C/1.2.3. where [sic] examined, they gave results, more irregular than, & inferior to those of the lens & pannel pieces reported above[.]

These results with the electric lamp shew generally what is the amount of divergence due to the condition & form of the surface of the glass, and they shew how much greater it is with the pressed than with the cut glass. The amount of divergence in a perfect lighthouse apparatus may be estimated approximately by the size of the flame & its distance from the refractors. To this divergence, which is generally & roughly estimated at about 6°, is to be added the divergence due to the glass which may make it 8°, 10°, 12° or more. All the glass divergence, even to the widest & fullest parts of the result, as with the upper & lower D pieces take away light from the chief place to be illuminated which we may consider as that within 6°; but speaking generally, for because of the irregularity of action it is only so I can speak, if we assume that the divergence with a lens is increased from 6° to 8° then the intensity of the light seen at a distance is diminished from 64 to 36 or by nearly one half;- if the divergence is increased to 10° the intensity is diminished from 100 to 36 or nearly by ⅔rds; and if increased to 12°, only one fourth of the intensity of the light remains, the rest being dispersed by divergence[.]

Defective action of the kind described increases as the focal distance is lessened, and decreases as that distance is greater; and as I understand that the cost of the pressed glass will be very much less than for the cut glass, it allows of the use of a larger size for a smaller one, by which the errors belonging to the smaller size may be reduced and also the size of the flame increased. Therefore, in harbour lights and those corresponding to the third & fourth order, the proposition, if it ultimately succeeds is more likely to be carried first into effect. Increasing the flame with increase of apparatus diminishes the proportionate effect of the glass disturbance; but with the accepted magnitude of a first class light, the magnitude of the flame cannot be well increased; because of the then necessarily increased size of the lenses & other circumstances. On the other hand attempts to exalt the force of the light by increasing its intensity & diminishing its size as in the Electric lamp & accompanying that by a diminished glass apparatus would increase the proportion of glass error.

These specimens of cast glass are the earliest produce of the attempt to simplify and economize the manufacture of the optical part of a lighthouse. They have not yet been extended to the service of high & leading lights, and much experience & progress will be required before a judgement can be formed of the probable final result. In the mean time they are very good & important as steps and deserve every encouragement; and there is no doubt they will have that in France where their application for trial & proof has already been commenced¹.

M. Faraday