344 
Proceedings of Royal Society of Edinburgh. [sess. 
The plane P and Q (fig. 4) in which the two spectra p and q 
are formed, is occupied by a screen whose function is to limit 
the field of view of the eyepiece to two narrow strips taken one 
from each spectrum for the purpose of having their intensities 
compared. This screen is shown diagrammatically in fig. 7. By 
means of the sliding piece A, the colour of the strip taken from 
the upper spectrum can be altered at pleasure, while by meaus of 
the second sliding piece a, mounted on the first, the width of the 
strip can be altered. The sliding pieces B and b perform similar 
offices for the lower spectrum. Through the opening of the slides 
the trimmer T may be seen. The latter is fixed at the side of the 
screen adjacent to the divided lens, and fulfils a function that has 
already been explained. 
After the screen there follows at a distance of about twice * its 
F 
[That some of the parts may be more easily seen, this diagram is not drawn to 
scale, nor does C show the true cross section of the lens at that place. ] 
focal length the divided spherical lens D (fig. 4), and, as shown 
above, the resulting images in the plane FF' (which is conjugate to 
the screen in the plane PQ) of the strips of the two spectra p and 
q can be arranged by adjusting the lens-halves so that their edges 
are in complete contact. 
It may be mentioned here that there is an alternative arrange- 
ment of the parts just described which has the merit of shortening 
the telescope tube. The latter point is important, because if an 
ordinary spectroscope prism be employed, all the parts of the 
* It will he recollected that the minimum distance between an object and 
the image of it formed by a convergent lens is equal to four times the focal 
length of the lens ; the divided lens has been placed at a distance of twice its 
focal length from the two spectra p and q (fig. 4), so that the telescope tube 
may be as short as possible. 
