THE I'RESKNT CONDITION OF ENGLISH OBJECT-GLASSES. 263 



glasses are spherically iinder-corrected ! ! 1 have long main- 

 tained the opinion that the glassesare purposely made to 

 show a certain standard effect, which effect is itself a delusion. 



But it is evident that when a miniature at the proper dis- 

 tance is found to be a precise, clear, and effective portraiture 

 of the reality, and bears large amplification Avith the utmost 

 satisfaction and clearness, that all doubt as to the goodness of 

 the glass is at once dispelled. 



If we ask the photograper the simple question — Will your 

 glass form with equal precision either a miniature of an ob- 

 ject or an enlargement of a small object? he will reply, that 

 depends upon the quality of the glass, but that a perfect glass 

 ought to do both for the distance at which it has been cor- 

 rected. It is surely just the same thing with the system I 

 have now been some time patiently advocating, viz. the mini- 

 ature test, enlarging it by the microscope. The goodness of 

 the glasses is proclaimed by the agreement of the enlarged 

 miniature with the object miniatured. 



The advanced photographer experiences none of those 

 bewildering stumbling-blocks in general which beset the ])ath 

 of the high-power microscopist, simply because he knows at 

 once whether his miniature resembles nature. If the fore- 

 ground and distance of a landscape is accurately and deli- 

 cately delineated on his plate, all in focus and without dis- 

 tortion, he can pronounce the result suj)erb at once. He 

 goes in general from the large to the minute. He can always 

 test his result by his known chosen object, every detail of 

 which he can examine and compare with his picture. 



But with the microscope all is changed ; the eye is indeed 

 photographing on the retina, as well as it can, the image of, 

 perhaps, an utterly unknown structure. We have no means 

 here of comparing the picture with the original. But if a 

 fine image one thousand times smaller will bear re-amplifica- 

 tion one thousand times, and reproduce the details of the 

 known original, the standard of correspondence, as in photo- 

 graphy, is at once rehabilitated. Still worse is it for the ob- 

 server with a preconceived notion of structure that he cannot 

 cast off the tyrant idea overriding his better judgment, even, 

 perhaps, against his will. 



The speculations on structure warmly disseminated during 

 the last twenty years will probably cause the microscopists 

 of the rising generation to compassionately smile upon the 

 credulity of their forefathers. 



I propose now to touch upon a few points connected with 

 the defining power of high-class objectives, which, I trust, 

 may not be uninteresting. 



