140 



THE PRESIDENTS ADDRESS. 



the late Professor Abbe, who first called so much attention to 

 it, laid so great a stress upon its observance. 



Lastly, there is yet another defect to remind you of. It is one for 

 which I claim your closest attention, for any little interest I can 

 arouse in your minds in what follows is largely associated with 

 what I have now to say, and so I wish you to grasp the situation 

 in its entirety. I refer to the subject of — 



Chromatic Aberration. — This — another name for the errors 

 which occur in the performance of lenses due to the different 

 paths pursued by light of differing wave-lengths — is a matter 

 of the greatest interest. 



Let us take first quite a simple lens, of short focus — such, 

 indeed, as you meet with in an eye-piece. Let us presume it is 

 mounted in a cell, so that we can attach it to the nose-piece of the 

 microscope, and that a piece of finely ground glass is placed over 



*\ HITE 



VVHITT 



UNACHROMATIC 



FisVlll 



OVERCORRECTION 



the eye-end of the draw tube, the ocular having been first removed. 

 A well-defined object being focussed on this glass screen — using 

 different apertures of the lens in succession, and employing mono- 

 chromatic light first of one colour and then of another — discloses 

 the fact that, owing to spherical under-correction, we only get 

 reasonably sharp images with a small aperture of the lens, and 

 generally owing to coma, only in or near the centre of the 

 field. ]f, with a small opening, we now try other colours of 

 the spectrum in succession, we find that an alteration of the 

 focal adjustment is required for each colour; and also that the 

 different coloured images, when in focus with a constant tube- 

 length, differ in size, for the red really requires the longest focus 

 and furnishes the smallest image, the violet having the shortest 

 focus, whilst it yields the largest image ; other colours in between. 

 This is better understood by examining Fig. 7. L is the simple 

 uncorrected lens ; E is a beam of white light entering it ; and Ei 

 the point where the lens, acting as a prism, breaks up the white 



