1902 - 3 .] Mr J. R. Milne on the Spectrophotometer. 
497 
looked at through the eyepiece of the telescope in the ordinary 
way, the two spectral strips of light a b and e d appear separated 
by the dark hand b c which lies between them. Let the eyepiece 
be removed, and an achromatic lens, divided into two symmetrical 
halves, be substituted, as shown at L L'. Each half of the lens 
will itself act as a complete lens, and will throw an image on a 
screen situated at S S'. Lines drawn through the optical centres 
0 O' show the positions of these images on the screen. By moving 
the lenses nearer together or further apart by means of slow- 
motion screws, the edges of these images on the screen S S' can be 
brought into contact. The images may be viewed from behind 
AB 
A'B 
a* 
through a translucent screen, prepared for this purpose according to 
the directions of Foucault,* the whole being enclosed in a tube 
attached to the spectrometer; or, if preferred, the images may be 
See Ann. de Chim. (6) vi. 342. 
