1905-1906.] Address by the President. 325 



cover-glass. The correction of the spherical and chromatic 

 aberration of microscopic object-glasses had now reached such 

 a stage that it was found in the case of object-glasses of high 

 power — say from l-4th or l-5th of an inch upwards — that 

 the cover-glass created an indistinctness in the image of the 

 object. The diagram on Plate XXVI., Fig. 1, will explain 

 the cause of this. 



Let M N be the front lens of an object-glass and the 

 object to be viewed. If there were no cover-glass, the rays 

 of light would proceed in a direct course from the object to 

 the object-glass. By the interposition of the cover-glass, C C, 

 a refraction of the rays takes place. Thus the marginal rays 

 enter the cover-glass at A and B, are refracted by it to E and 

 P, and on emerging take a course parallel to their former 

 course and enter the object-glass at M and N. But, accord- 

 ing to optical principles, these rays appear to proceed from the 

 point Y. On the same principle more central rays, as S T, 

 appear to proceed from the point X, and so on till the central 

 ray, which, as it does, appears to proceed from the point 0. 

 This, of course, produced indistinctness in the image of the 

 object viewed. 



Collar Adjustment. — Opticians now set about to devise a 

 remedy for this indistinctness. One course suggested was 

 to make all the cover-glasses of a uniform thickness and to 

 correct the object-glasses for that thickness. This was, how- 

 ever, recognised to be unworkable. Mr Andrew Boss hit upon 

 the correct means, and published them in the ' Transactions of 

 the Society of Arts' for 1837. Put shortly, it was to vary 

 the distance between the front and back lenses of the object- 

 glass. This is effected by means of a collar. As has been 

 said, object-glasses usually have three pairs of lenses : the 

 front pair cannot be moved, the second pair is in a setting 

 inside of the outer tube, and the third pair in an innermost 

 setting. The settings of the second and third pairs are so 

 joined that the turning of the collar moves them together, and 

 makes them approach to or recede from the front pair. In the 

 English form, especially of older date, the word " uncovered " 

 is engraved on the tube ; and in viewing an uncovered object 

 the collar has to be set accordingly. When the object is 

 covered the collar has to be turned so as to make the back 



