From Richard Thomas Lewis   March 25, 1862¹

To Professor Tyndall

1 Lowndes Terrace, Knightsbridge, S.W.,

March 25, 1862.

Sir,

The enlarged appearance of the sun and moon at rising or setting has long been a subject of controversy; and although astronomers and others have proved by micrometrical measurements that the apparent diameter of those bodies is actually largest when they are on the meridian (since they are nearest to us then), the popular mind refuses to believe that the illusion is only mental, and hitherto no illustration has been found sufficiently satisfactory to convince those who are sceptical in the matter. The best illustration brought forward as yet, to my knowledge, was that which appeared some time since in ‘Recreative Science,’² in an article upon “The Rising and Setting of the Sun Physiologically considered”. The writer had, one dark night, been gazing for some time upon the glowing furnace of a lime-burner, after which, on going out into the dark, he saw the luminous image of the fire in the air before him as a large circle; but happening to look up, this image apparently shrank up to about half its former diameter. This, however, does not appear to satisfy many persons; and since your most interesting lecture of the 16^th ult.³ was the means of my discovering a far more forcible and complete illustration of this singular effect, I venture to trespass upon your time by detailing it to you. On my return home after that lecture I proceeded to make sundry experiments upon binocular vision, amongst which were the following:—

Centring the eyes upon a distant point A, I introduced a stereograph of the moon at such a distance from the eyes that the axis of each passed centrally through the picture taken for it; the result was a perfect and beautiful stereographic projection of the moon at A, apparently solid and spherical (as fig. 1). Then, without in any way altering the distance between the eye and the slide, I centred the [INSERT IMAGES 1871a eyes upon a point nearer to me than B, in such a way that the & 1871b] axis of the right eye passed through the centre of the left picture, the left also passing through the right: the result was a perfect view of the moon, but in this case inside out, as you termed it (the moon appearing quite concave), the image being seen in the air at C, and appearing in a most perceptible degree smaller than when in the former case I saw it apparently at A, nearly double the distance. Now as the distance between the real object and the eye was in both cases identically the same, the image upon the retina must have been of the same size in each experiment, the apparent enlargement of the object in one case and contraction in the other being purely mental. This singular effect may have been noticed by yourself before this; but in case it should not have come under your notice, I thought you would perhaps excuse the liberty I take in addressing this communication to you. Of course it would be quite out of place and equally unnecessary for me to explain the philosophy of the matter here, or the difficulties connected with first attempts to break the eye of the habit of focusing itself according to the amount of convergence; but for the delight with which I, as a young student of astronomy, made the above observation, as also for my future independence of all stereoscopes, I feel greatly and entirely indebted to the course of lectures upon light. In performing the second of the two experiments named, so perfect a perspective inversion is seen in the case of landscapes, that one feels for the moment endowed with the fabled optical powers of the lynx, men being seen most distinctly through brick walls, houses, or even hills.

Hoping, Sir, that you will excuse the liberty I have taken I thus addressing so long a communication to you,

Believe me to be, Sir| Yours very respectfully| Richard T. Lewis.

‘On the Changes in the Apparent Size of the Moon’, Phil. Mag., 23 (1862), pp. 380–82.