186 Transactions of the Society. 



Take the Podura scale : I can give you an image of it with my 1/25 in. 

 and 1/50 in. of twenty years ago. What I, in common with most 

 microscopists, considered then the best result, the most sharp, clear, and 

 delicately defined image, with those lenses, I can get now ; but with those 

 objectives, nothing better. 



But the elements — the essential features that constituted the quality 

 of beauty in that image — are the very elements, the actual features, that 

 every admitted improvement in our object-glasses has brought out more 

 perfectly. So that if I now put, say the Podura scale under my old dry 

 1/25 in. objective, and beside it another precisely similar scale under a 

 new homogeneous 1/20 in. objective of N.A. 1 • 5, the very qualities of the 

 image which I, and experienced microscopists generally, thought the best 

 twenty years ago, are incomparably transcended in beauty and perfectness 

 now. 



But that is not, and has not been, my only or my chief test. It has 

 been one eminently practical so far as my own work went — at least for 

 some years. 



Up to ten years ago, although I had spent weeks in patient effort, 

 no lens that I possessed or that was within my reach could be made to 

 reveal the flagella of Bacterium termo. The flagella of many minute 

 monads, and of such bacterial forms as Spirillum volutans, and even 

 Bacterium lineola, I could demonstrate ; though some of them with 

 difficulty, but not a trace of those of B. termo. But near that time 

 Powell and Lealand produced a battery of immersion lenses on a new 

 formula, and of much relative excellence ; and with these lenses the 

 flagella of B. termo were brought within the range of sight. 



Since that time, that has been a good lens to me, in proportion to the 

 greater or less ease and perfection with which it has revealed this 

 delicate fibre. And let me say, that such lenses as do this are those 

 that always, without fail, give us the best ideal image of Podura scales 

 and other tests. You will pardon me, I trust, for this amount of personal 

 reference, since it will give a greater relevancy to what will follow. 



Improvements of great optical importance have been made during 

 the last few years. The manufacture of homogeneous lenses by Messrs. 

 Powell and Lealand, gave us the opportunity which we could not have 

 with foreign makers, of urging certain modifications. The addition of 

 the correction collar was a minor, but still important item. But the 

 great point was the increase of the N.A. These makers have shown 

 themselves most anxious, and have spared no efforts to reach the highest 

 aperture yet attained. 



Advancing, say, from N.A. 1-25 they attained to 1*38 in such 

 powers as the 1/25 in. and the 1/50 in.; subsequently to 1*47 in a 

 1/8 and 1/12 in. objectives, and finding these from my working point of 

 view of such supreme gain, I urged them still on ; and was ultimately 

 rewarded by the possession of a 1/G in., N.A. 1 • 5, followed by a 1/12 and 

 a 1/20 in. foci of the same great aperture. From each of these I obtained 

 special advantages over all equal powers, but with lower apertures, within 

 my reach. 



A question frequently asked may be asked again, in what way these 

 last increments of aperture aid us. The practical answer is not difficult. 



