1903 - 4 .] Mr Milne on a New Form of Spectrophotometer. 345 
optical train that follow must be capable of rotation about a 
vertical axis through the prism to an extent sufficient to cause 
all the various colours of the spectrum in their turn to fall upon 
the eyepiece and to be seen by the eye of the observer. In the 
usual form of spectrophotometer this is achieved by supporting 
the telescope only at its objective end, which is pivoted so as to 
have the required rotatory motion. Now in this instrument the 
telescope tube must have a length equal to the focal length of the 
cylindrical telescope objective, plus a further length, equal to four 
times the focal length of the divided lens. Such a length of tube 
makes it difficult to secure the necessary rigidity without resorting 
to a cumbrous form of mounting. 
The alternative form of apparatus just mentioned, which is 
shown diagrammatically in fig. 5, is provided with a single lens, 
C and D, which takes the place of the two lenses C and D 
of fig. 4, with the result that the telescope tube is shortened 
by a length equal to the distance between the planes PQ and FF'. 
In order to find the specification of the lens required in this case 
two points must be borne in mind. In the first place the lens, 
when placed behind the prism R (fig. 4), must give rise to two 
pure spectra formed from the two beams of light respectively. 
Now a cylindrical lens with its axis of figure upright will fulfil 
the above condition. Its focal length may equal the distance 
from C to the line PQ, so that the spectra will be formed in a plane 
normal to the paper through the latter line. In the second place, 
as already explained, to avoid diffraction effects the trimmer must 
be situated in a plane conjugate to that in which the spectra are 
formed. To fulfil this condition along with the other the lens, 
having its front face ground to the cylindrical curvature deter- 
mined above, must have its back face ground as a cylindrical 
lens whose axis of figure is horizontal. The exact focal length 
of the curvature on the back face of the lens we shall discuss 
later. At present it will merely be specified that it is to be less 
than the focal length of the curvature formed on the front face. 
This lens will bring the beam of parallel light ABCD (fig. 6a) to 
a line focus EF, where EF is situated as before at a distance from 
the lens equal to the distance, from C to the line PQ (fig. 4). 
Before reaching EF, however, the beam is first brought to 
