188 Transactions of the Society. 



high quahty of their immediate predecessors, would have given a very- 

 elevated standard of comparison ; and the result was that, after most 

 exhaustive and critical investigation with the same tests, the potentiality 

 of the system represented by the apochromatic lens, most powerfully 

 and hopefully impressed me. I felt, in fact, that the lens itself was of 

 great merit. But withal, by the standard of test the latest of my 

 lenses enabled me to employ, I felt that its merits had been over- 

 estimated. 



It is quite true that on some of my delicate test objects, the images 

 shown by the apochromatic lens, in combination with the " compensating" 

 eye -pieces, appeared to advantage, when compared with my lenses com- 

 bined with the ordinary eye-pieces ; but when I tried my own various 

 powers with the same compensating eye-pieces, I am constrained to say, 

 that no real advantage over my latest lenses could be discovered. My 

 judgment therefore was most favourable as to the immense advantage of 

 the eye-pieces, and of the possibilities that lay in the entire system, 

 rather than in this special apochromatic object-glass taken by itself; 

 and although pressed again and again by the editors of journals to give 

 a public expression of my judgment, I steadily declined, feeling that it 

 was not, and could not at that time be exhaustive. 



Later, an opportunity was courteously afforded me, by the makers, to 

 examine a complete series of these object-glasses, from 1 in. to 1/8 in. 

 focus, and with eye-pieces fitted for English stands. 



In the examination of these objectives and their systems of eye-pieces, 

 I spared no pains to be exhaustive and impartial. I desired to find the 

 evidence of progression in optical excellence for which I am always in 

 search, and the excellence of the 1 in. greatly impressed me ; but I failed 

 to realize my high hopes in the behaviour of the higher powers. The 

 result, however, of a most critical examination was to very greatly 

 strengthen my conviction of the value of the optical system which these 

 lenses represented, and above all, of the excellence of the actually new 

 resource provided for us by the compensating eye-pieces. 



In what I have here said I must again remind you that the com- 

 parison of Zeiss's apochromatic object-glasses was with a group of 

 object-glasses the most carefully made, most excellently corrected, and 

 with the widest numerical apertures, of any object-glasses that had 

 ever passed through my hands, based on the old system of correction. 

 But with this understanding, it appears to me a responsibility that I 

 must not evade to state the facts at this crisis in the development 

 of object-glasses. And I do this with the more confidence, that, as I 

 have already informed you, Mr. Mayall, wholly independently of me, 

 examined tlais set of objectives and eye-pieces, and we each recorded 

 separately in writing our judgments at the time of examination ; and 

 I subsequently found that our resulting judgments were almost identical. 



During this time samples of the new optical glass had reached the 

 English opticians, and Messrs. Powell and Lealand, in a relatively brief 

 time, and on a formula of their own, made an apochromatic 1/12 in. 

 object-glass and eye-pieces, constructed on the plan devised by Abbe. 

 By the wise advice of Mr. Mayall this was exhibited at our November 

 meeting. My high opinion of that lens and its compensating system of 



