TO MISS CAY.

                                                                       Trin. Coll., Whitsun. Eve, 1854.

I am in great luxury here, having but 2 pups., and able to read the rest of the day, so I have made a big hole in   some subjects I wish to know. We have hot weather now, and I am just come from a meeting of subscribers to  the Bathing Shed, which we organised into a Swimming Club so as to make it a more sociable affair, instead of  mere "pay your money and use your key."    A nightingale has taken up his quarters just outside my window, and works away every night. He is at it very fierce   now. At night the owls relieve him, softly sighing, after their fashion.

I have made an instrument for seeing into the eye through the pupil. The difficulty is to throw the light, in at that  small hole and look in at the same time; but that difficulty is overcome, and I can see a large part of the back of the   eye quite distinctly with the image of the candle on it. People find no inconvenience in being examined, and I have   got dogs to sit quite still and keep their eyes steady. Dogs' eyes are very beautiful behind, a copper-coloured   ground, with glorious bright patches and networks of blue, yellow, and green, with blood-vessels great and small.
 

Trin. Coll., 24th Novr. /54.

I have been very busy of late with various things, and [209] am just beginning to make papers for the examination  at Cheltenham, which I have to conduct about the 11th of December. I have also to make papers to polish off my   pups. with. I have been spinning colours a great deal, and have got most accurate results, proving, that ordinary   peoples' eyes are all made alike, though some are better than others, and that other people see two colours   instead of three; but all those who do so agree amongst themselves. I have made a triangle of colours by which   you may make out everything.

You see that W lies outside the triangle B, R, Y, so that White can't be made with Blue, Red, and Yellow; but if   you mix blue and yellow you don't get green, but pink—a colour between W and R. Those who see two colours   only distinguish blue and yellow, but not red and green: for instance—

6 of blue and 94 of red make a red which looks to them like a gray made of 10 W and 90 Black.

 40 of blue and 60 of green make 34 of W and 66 Black.

 I should like you to find out if the Normans have got Bishop Percy's Reliques of Ancient Ballad Poetry, for if   they have I would not send them a duplicate; if not I think the book would suit one-half of that family.

If you can find out any people in Edinburgh who do not see colours (I know the Dicksons don't), pray drop a hint   that I would like to see them. I have put one here up to a dodge by which he distinguishes colours without fail. I   have also constructed a pair of squinting spectacles, and am beginning operations on a squinting man.