From John Herschel   Nov. 16/64

Collingwood | Nov. 16/64.

My dear Sir

I am much surprised at your failure to get my heat spots¹ α β χ δ ε. I have no idea that they can be of atmospheric origin—at least, if so, not arising from any variable or accidental states, as I have never failed to produce them whenever there was good sunshine.

I always imagined that they were due to some peculiarity in the scale of transcalescence of some one or more of the glasses used—and for that reason I had projected to get both lenses and prisms of Rock Salt but I never could procure a sufficiently large & clear piece to work a lens of sufficient diameter.

The prism I used was a Fraunhofer² one of Munich glass the lens an achromatic of 3 inches 1/8 clear aperture and about 24 inches focus by Cauche³ and I never failed to get the spot nor did I ever perceive any difference in their relative magnitude other than might be fairly attributed to greater or less intensity of sunshine—or in their places in relation to the visible rays of the spectrum.

I have the glasses all still by me and if you would like to try them I will send them to you.

I have tried to photograph the spots permanently by using coloured alcoholic solutions and applying repeated washes so as to concentrate the coloring matter more on the dried or partially dried places than on the general ground—but could not succeed in getting any good results worth preserving—though there were faint images so imperfect –but not sufficiently disengaged from the general colour.

I remain | Dear Sir | Yours very truly | J. H. W. Herschel

RI MS JT/1/H/102

RS HS/396

RI MS JT/1/TYP/2/532