162 



THE AMEEICAN MONTHLY 



[September, 



retical results — in the history of the 

 microscope. He referred to the inves- 

 tigations of Prof. Abbe, as having led 

 to a knowledge of the principles of mi- 

 croscopical optics. In the line of 

 delicate investigation Messrs. Dallin- 

 ger and Drysdale have watched the 

 growth and development of objects 

 so inconceivably minute as to have 

 been heretofore regarded as invisible. 

 They have observed flagella of a di- 

 ameter one-tenth the breadth of an 

 absolutely immeasurable line. 



American microscopists, he said, 

 are now going over the track which 

 the English passed over twenty or 

 twenty-five years ago, and have now 

 abandoned. As we are going now, 

 they went and found no valuable re- 

 sults to biological research. He had 

 lived through the whole period of 

 achromatic microscopy in England. 

 About 1828-9, when nothing was 

 known there of what had been done 

 in France, or by Amici, Goring 

 stimulated Tulley to make an achro- 

 matic lens of ^-inch focus. I saw 

 that and was very much impressed by 

 it. In London I bought a French 

 combination of Chevalier, and ever 

 since I have had a microscope — for 

 over fifty years. The first advantage 

 recognized in the use of achromatic 

 lenses was that the angular aperture 

 could be increased. With a single 

 lens a stop or diaphragm had to be 

 used, cutting off the marginal rays, 

 but it was soon found that with an 

 achromatic lens the whole face of the 

 lens could be used, and thus more 

 light and finer definition were ob- 

 tained. At first the scales of the blue 

 Menelaus were used as the finest test ; 

 then, when a quarter-inch was pro- 

 duced, we began to see parallel, wavy 

 markings on the podura scales, which 

 was then regarded as a great feat. 

 After that came the definition of the 

 markings, as we see them at the pre- 

 sent time. I still hold the podura scale 

 as the best test of an objective for 

 biological research. 



For a long time there was a rivalry 

 between Ross, Powell & Lealand and 



James Smith as to which should make 

 the widest angular apertures. As 

 these were increased the diatom-tests 

 came into use. Sollitt discovered 

 some diatoms which he used as test- 

 objects, and it was soon found that 

 resolution depended upon angular 

 aperture and less upon defining 

 power. The biologists still held to 

 the podura scale test — they said a 

 lens should not only resolve dia- 

 toms but also define podura. The 

 podura should not be neglected. But 

 the manufacturers did not take the 

 same view of the subject, and they 

 continued to make lenses to resolve 

 diatoms. The introduction of dia- 

 tom-tests was, however, exceedingly 

 valuable. But, besides definition, fo- 

 cal depth is required. I have seen it 

 stated in some American and French 

 publications that any penetration in 

 a lens is a great fault. I hold that a 

 lens which does not possess that 

 quality is inferior. 



Within the last few years we have 

 come to understand many things. 

 Prof. Abbe has shown that resolution 

 is not effected in the manner hereto- 

 fore supposed — that diffraction spec- 

 tra take part in resolution. Now we 

 understand why angular aperture is 

 valuable. Prof. Abbe has come to 

 the same conclusions I have advocat- 

 ed for thirty years — that it is impos- 

 sible to have focal depth and very 

 wide angular aperture. Increased 

 angle has given great power of reso- 

 lution ; but what else ? Nothing at all. 

 Angle can only be obtained by les- 

 sening working distance. The result 

 is, we see nothing but what is in the 

 focal plane. It is well and service- 

 able to apply such lenses to verify a 

 thing you have already worked out. 

 Drs. Dallinger and Drysdale used a 

 power of 4,000 diameters, and watched 

 the swelling out and growth of mere 

 unmeasurable particles. This is high 

 biological work, and Mr. Dallinger 

 assures me that it could not be done 

 without penetrating power. Mr. Dal- 

 linger is perfectly sensible of the 

 value of different kinds of objectives 



