63 LECTURES TO SCIENCE TEACHERS. 



you can work with ordinary daylight. Suppose you have 

 a room which you can darken, and that you are at liberty 

 to cut a hole four or five inches square in the shutter, under 

 which it is convenient to screw on a ledge for the sake of 

 supporting the object to be examined, and that you cover the 

 hole by a suitable glass. The most useful generally is a 

 dark blue coloured by cobalt, or a dark violet coloured by 

 manganese. The blue does better for some things and the 

 violet for others. Cover the hole in the shutter with the 

 deep coloured glass or, as is occasionally better, with a 

 solution of a salt of copper, such as the nitrate, which is 

 more convenient than the sulphate on account of its great 

 solubility. Suppose you have daylight filtered as it were 

 through the deep blue or violet glass ; then if you place 

 in front of the glass a test tube containing a solution of 

 quinine in dilute sulphuric add, or a solution obtained from 

 the bark of the horse-chestnut with a little ammonia, you 

 will see this blue phosphorescent looking light to perfection. 

 But supposing you have not a window-shutter which you 

 can make a hole in, still you can get on very well with an 

 old packing-case. You knock off part of the top of it, so 

 that you may look in, and saw off a portion obliquely 

 parallel to the opposite top edge and on the slanting rectan- 

 gular hole thus formed you nail a piece of board, making 

 a window in it four or five inches square, with a little 

 ledge to keep the glass with which you cover it from 

 slipping down You place it near the window and cover 

 your head with a dark cloth as if you were looking into a 

 camera obscura, and so you can see the phenomenon to per- 

 fection. 



If you want to demonstrate that it is really this phenomenon 

 you are dealing with, it is desirable to have a second glass 

 in a certain sense complementary to the first. If we could 

 pick out media which absorbed light just in the way we 

 wished, we should choose a coloured glass perfectly opaque 

 from the red end up to the blue or violet, and perfectly trans- 

 parent beyond, and a second glass perfectly opaque for those 

 rays for which the first was transparent, and vice versa. 

 But as we cannot make media to command to absorb what 

 parts of the spectrum we like, we must make use of the 

 best which the colouring matters of nature afford us ; and 

 if you take a blue glass and a yellow glass they will in 



