468 



NA TURE 



[^iVIarch 17, i8«7 



Abbe's method, I find that in practice this amount may 

 be deducted as of very little service, in all apertures 

 beyond about l'3 ; hence, to be able to utilise fully any 

 given aperture beyond rj, it is practically necessary that 

 the measurement by means of Abbe's apertometer should 

 be about 10 per cent, higher. 



But a further advantage of great numerical aperture is 

 that, other things being equal, we can utilise with excellent 

 results deeper eye-pieces. 



I have long realised the advantage, with finely corrected 

 objectives, of a far larger series of eye-pieces than the 

 catalogues provide. Messrs. Powell and Lealand several 

 years ago made me one or more eye-pieces between each 

 of their deeper eye-pieces of standard catalogued focus, 

 and they certainly, within the limit of excellence beyond 

 which greater eye-piece power cannot be employed, bring 

 out to far greater perfection the qualities of any high-class 

 object-glass. 



But we have had announced to us an improvement in 

 the optical arrangement of the microscope, based upon 

 an important and fundamental change in the media em- 

 ployed in the construction of object-glasses and eye- 

 pieces. It will be known that I refer to the system of 

 apochromatic object-glasses and compensating eye-pieces 

 devised by Prof. Abbe, and under his auspices carried out 

 by Messrs. Zeiss, of Jena. 



The aim of the construction of these new objectives 

 and eye-pieces has been to provide a higher degree of 

 achromatism than could be reached by the old media ; 

 the new kinds of glass produced at the Jena Optical 

 Glass Works, under the superintendence of Dr. Schott 

 and Prof. Abbe, can be so combined in the construction 

 of an object-glass as to achromatise not only the essential 

 portion of the primary spectrum, but also to a great ex- 

 tent the secondary spectrum, leaving only small residuals 

 of the tertiary order still visible under certain test con- 

 ditions. The final elements of correction are supplied 

 by "compensating" eye-pieces of special construction 

 designed to correct what Dr. Abbe refers to as "the 

 differences in the amplification of the image for the 

 various colours . . . formed by the objective outside the 

 axis, which cannot be corrected by the objective itself. ' 



The first trials of these new optical combinations made 

 in Germany evoked unstinted praise, and those who, like 

 myself, desired nothing so much as real improvement, 

 awaited their arrival in England with eager and even 

 anxious curiosity. 



The first that came to this country came to Mr. Frank 

 Crisp, and by his courtesy this lens, an apochromatic of 

 1/8 inch focus was placed in my hands. 1 subjected it 

 to comparison, in succession, with my complete set of high 

 powers, including those of N.A. 1-5, and upon tests, and 

 by methods, which 1 have indicated. 



It will be well understood that the high excellence and 

 great aperture of my three latest object-glasses would 

 have given a very elevated standard of comparison, a 

 standard of comparison, so far as I know, never before 

 reached, and the result was that with the potentiality of 

 the system represented by the apochromatic lens I was 

 most powerfully and hopefully impressed. I felt, in fact, 

 that the lens itself was of great merit. But withal, by 

 my standard of test 1 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 combina- 

 tion with the " compensating " eye-pieces appeared to 

 advantage when compared with my lenses combined with 

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

 powers with the same compensating eye-pieces I am con- 

 strained 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 the 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 judg- 

 ment, 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 i-inch to l/8-inch focus, and with eye-pieces 

 fitted for English stands. In the examination of these 

 objectives and their system 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 1 

 am always in search, and the excellence of the i-inch 

 greatly impressed me ; but 1 failed to realise my high hopes 

 in the behaviour of the higher powers. The result, how- 

 ever, 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 comparison ofZeiss'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 responsi- 

 bility that I must not evade to state the facts at this crisis 

 in the development of object-glasses. And 1 do this with 

 the more confidence that, as 1 have already informed you, 

 Mr. Mayall, wholly independent of me, examined this set 

 of objectives and eye-pieces, and we each recorded separ- 

 ately, in writing, our judgments at the time of examination ; 

 and I subsequently found that our resulting judgments 

 were almost identical. 



Dur ng this time samples of the new optical glass had 

 reached the English opticians, and Messrs. Powell and 

 Lealand in relatively brief time, and on a formula of their 

 own, made an apochromatic 1,'12-inch 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 eye-pieces I at that 

 meeting expressed, and need only add that since I have 

 become the possessor of a second object-glass of precisely 

 similar construction and power made by the firm, I am 

 much strengthened in the opinion I gave. 



We all appreciate the splendid services rendered to 

 microscopy by Prof. Abbe, and it was a happy expression 

 of that appreciation that led Mr. Mayall to propose a 

 visit to Jena, with his microscope and such object' 

 glasses as he thought would worthily represent the stand- 

 point we had now reached in England. 



I understand that Prof. Abbe greatly desired this, 

 wishing to possess the fullest information as to our 

 methods of testing object-glasses, and to be permitted to 

 examine our best optical work. 



I need hardly say that it was a source of great pleasure 

 to me to place at Mr. Mayall's disposal all the lenses and 

 apparatus I possessed that would serve him ; for it was 

 in the highest interests of the microscopy of the world 

 that so great a leader in recent progress should see the 

 eflects of his teaching and practice as evidenced by our 

 latest object-glasses, and especially by the new apochro- 

 matic i/i2th by Powell and Lealand, with its system of 

 compensating eye-pieces. 



Mr. Mayall has told us the story of his visit: of his 

 kindly reception ; of the earnest and repeated trials of 

 the object-glasses he was able to submit to Prof .Abbe ; 

 and of the frank appreciation expressed by Prof. Abbe of 

 the English object-glasses. This comparison will, in my 

 judgment, "make history "for the future of our instru- 

 ment. It will react here and in Germany. Prof. Abbe's 

 splendid powers are more than ever concentred on the 

 work of touching a higher perfection in object-glasses ; 



