76 Immersion Lenses and New Befradometers. 



I have submitted tlie very finest immersion objectives now 

 known to tbe image test, and, as might reasonably be supposed 

 ■with all human workmanship, I find the residuary error small, but 

 still appreciable. The greater the angular aperture, the greater 

 rises the almost insurmountable difiiculty of compelling the oblique 

 pencils to converge to exactly the same focal point as the more 

 central rays ("within a ring much less than the 50,000th of an inch). 

 Still, the best glasses are a marvel of skill, worthy of the science of 

 the nineteenth century : the least circle of aberration being so ex- 

 ceedingly small. No first-rate optician claims absolute perfection. 



I think it right to add to this paper that I possess an inch 

 objective which displa^js the heading of the Macrotoma podura, and 

 which I had the honour of exhibiting to Dr. Lawson on the 14th 

 instant (with about 600 diameters) — in order to avoid imaginative 

 effects — before he had seen them with the half-inch of Wray, and 

 the ^th, and yVth immersion of Powell and Lealand. 



I am unwilling to retire from this controversy without recording 

 an example of Mr. Wenham's " fair spirit " — or rather spirited — 

 attacks upon these papers (and highly suggestive) : — 



" Our thanks are due to Dr. Pigott for his laudable attempts to 

 advance the microscope object-glass ; but as he appears to stand 

 forward as the pioneer of a new era in their construction, ignoring 

 all glasses made before the publication of his first essay as ' old- 

 fashioned,' such dictatorship naturally challenges inquiry as to the 

 merits of his investigations, and whether they have in any way con- 

 tributed to the end in view " (p. 16, 1871). 



Whether object-glasses are perfect, as Mr. Wenham guarantees 

 them, — whether any error can be detected, and whether the glasses of 

 1871 are finer than they were in 1869, 1 leave to the decision of com- 

 petent adjudicators, free from prejudice, and free from compromised 

 opinions. If my declaration of the detection of the 50,000th of an 

 inch aberration in the best glasses is thought so prodigious, I must 

 exercise greater circumspection so as not to offend susceptibilities 

 as unwonted in delicacy as conspicuous in refinement. I now 

 consider the controversy has extended long enough to tire the 

 patience of our Fellows, whilst the tone adopted creates an impera- 

 tive necessity for bidding it adieu in pages devoted to the ' Trans- 

 actions of the Koyal Microscopical Society.' 



As there is an emulation of no common kind between American 

 and English microscoj)ists, the following passage from the ' American 

 Cyclopaedia,' vol. xi., " Microscope," 1861, is not without interest. 

 The article claims superiority for American glasses. Speaking of 

 the Amphipleura peUucida, 130,000 hues to the inch, it says : — 



" Mr. SolHtt and Mr. Lobb both claim to have resolved it from 



