362 On the Measurement of the [July, 
image of the selenite plate in front of the aperture I will be divided 
by K into a red and a green disc. 
Fia. 2, 
Fig. 2. shows the instrument fitted up. A is the eye-piece 
shown in enlarged section at Fig. 8. G B is a brass tube, blacked 
inside, having a piece, shown separate at D C, slipping into the end 
B. The sloping sides, D B, B C, are covered with a white reflect- 
