282 the president's address. 



from the upper focal plane of the objective. Further, I might 

 be allowed to state, whatever be the length selected — whether 

 1G0 or 180 mm. — the actual plane of focus is called the "image 

 plane." 



Years ago there was no fixed limit of length for the optical 

 tube, one maker taking one length and another a different one. 

 What was equally troublesome, there was no fixed distance at 

 which the eye-piece should drop into the draw-tube, so that its 

 lower focal plane should coincide with the image plane, just 

 mentioned, of the objective. In consequence of this unsettled 

 state, a trouble arose that an objective by one maker did not 

 appear to magnify so much as, or perhaps it magnified more 

 than, one of the same focus made by another firm. Obviously, if 

 the eye-piece dropped in too far, it was like shortening the optical 

 tube-length, because the image plane must be lowered by a 

 change of focus so as to coincide with the lower focal plane of the 

 ocular. I have already explained that shortening the optical 

 tube-length means lessening the magnification, and hence the 

 cause of the trouble. If the reverse — the eye-piece not dropping 

 far enough — then it was the same as increasing the optical tube- 

 length, for the image plane had to be raised in this instance to 

 reach the lower focal plane of the ocular, which meant increasing 

 the optical tube-length, and of course the magnification. 



All these troubles were set right by the great ability of the 

 late Professor Abbe. When he first took the subject in hand, 

 everything was in a state of chaos and confusion ; there was 

 nothing definite. It was he who laid down the laws (at any rate, 

 for his new apochromatic lenses) that the optical tube-length 

 should be 20 mm. longer than the mechanical, whether English 

 or Continental, and that the lower focal plane of every com- 

 pensating eye-piece should be so computed as to lie 12 mm. below 

 the mouth of the draw-tube. The direct outcome of this capital 

 idea is that the compensating eye-pieces, when interchanged, do 

 not require any change of focus save to a very limited amount, 



