DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
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front; (3) with telescope and collimator holders at 45° to the frame; and (4) with a 
holder for a thinly silvered piece of optically plane glass, 4X2 inches, at intersection 
of axes of telescope and collimator. 
The drawings in Plate 31 sufficiently illustrate this part of the apparatus. 
The telescope and collimator were apair (l^-inch aperture 1 foot focal length) given 
to the Univ. Coll., Liverpool, by Mr. I, Roberts, F.R.S. They happened to have 
quartz lenses, which was unnecessary, but otherwise were well adapted for the 
purpose ; the slit or aperture of the collimator having especially convenient motions. 
To the eye end of the telescope, in addition to its own cross-wire eyepiece of low 
power (which was useful for setting), I adapted an excellent micrometer by Cooke, 
belonging to a 4-g-inch telescope, presented to the College by Mr. George Rogerson.^ 
It has a pair of independent micrometer heads, each divided into 100 parts, moving 
respectively a vertical spider line and an X* if also has eyepieces of various powers : 
the one commonly uised for the measurements here recorded being marked “ 200.” 
Fig. 
7, 
Diagram showing images in simplest case, with three mirrors, and beam going only once round optical 
square. The points to be imaged are the splitting points on semi-transparent mirror O. Two such 
points are shown, one imaged by a dot, the other by a ring. A represents the pair of images in 
first mirror for the transmitted ray; B the image of these in second mirror; C the image in 
mirror 3. Dashed letters mark the corresponding images for the reflected ray; and the final 
function of the semi-transparent mirror is to make C' coincide with C so as to give interference. 
Examining the path of a beam of light from collimator to telescope round the 
frame, as shown most simply in fig. 7, it appears that, by reason of there being an odd 
5 D 2 
