1905-6.] Exception to a Certain Theorem in Optics. 
527 
On an Exception to a Certain Theorem in Optics, with an 
Application to the Polarimeter. By James Robert 
Milne, B.Sc., Carnegie Research Fellow. 
(Read July 16, 1906. MS. received October 15, 1906.) 
[Abstract.] 
There is a well-known law in geometrical optics, that the 
“ intrinsic luminosity " of the image formed by any lens system 
whatever is the same as the intrinsic luminosity of the object. An 
exception, however, which seems not to have been pointed out 
before, exists in the case of polarised light, based on the fact that 
by the agency of a double-image prism two light rays polarised in 
directions mutually perpendicular may be combined into one ray, 
which carries the total energy of both. In this way an intrinsic 
luminosity of image can be attained which is twice as great as 
that of the object. 
Now, because the iris opening of the eye is of a fixed size, the 
D 
Fig. 3. 
only way of increasing the brightness of a retinal image is to 
increase its intrinsic luminosity; hence it is in connection with 
images formed in the eye that the above principle has its chief 
interest. 
The application to the case of the polarimeter is shown 
diagramatically in fig. 3. A is the polarising Nicol ; B the tube 
for the liquid ; C a Nicol half-covering the field as in Lippich’s 
half-shade device (or instead, there may be used the half-shade 
