The President's Address, 81 



series of American photomicrographs of the Podiira scale, in 

 which the best results were obtained with Powell and Lea- 

 land's voth, then with Wales' ^th and amplifier, and Wales' 

 r.^th immersion lens. Hartnack's No. 11 immersion lens 

 did not give a good result, which Dr. Woodward thinks 

 might have resulted from the great want of coincidence of 

 the visual and chemical rays, but which Dr. Maddox is dis- 

 posed to ascribe to some triflng error in centering when the 

 necessary chemical correction was made. 



In the course of a recent discussion concerning the com- 

 parative merits of English and continental objectives, there 

 has been a disposition, in some continental quarters, to con- 

 demn the use of deep eye-pieces, and this fact points to the 

 imperfection of the continental objectives. An English 

 microscopist invariably tests his objectives with deep eye- 

 pieces, and condemns those which Avill not stand the trial. 



A first-rate glass will perform much better with a B or C 

 eye-piece than a second-rate one with an A eye-piece ; and it 

 is often extremely convenient to use a lower power with a 

 deeper eye-piece in preference to a higher power with a 

 lower eye-piece, as the former method gives a greater work- 

 ing distance between the lens and the object, and a greater 

 degree of penetration — that is, presuming the lower objec- 

 tive has a smaller angle of aperture than the higher one. 



No continental maker exhibited any microscope stands 

 possessing the finish or the mechanical advantages of our 

 first-class instruments ; but a cheap form, devised by Nachet, 

 was found to be meritorious and convenient, having an ex- 

 cellent rotating stage, a point which Dr. Carpenter — than 

 whom there can be no better authority — considers essential 

 to the best working of a binocular instrument, as, without it, 

 it is often impossible to bring an object into the most advan- 

 tageous position with regard to the light. 



There is, however, one point to which I wish to direct 

 attention, and that is, the excellence of some of the French 

 objectives corrected for immersion; that is, introducing a 

 drop of water between the covering-glass of the object and 

 the outer surface of the objective. 



This plan, as Dr. Maddox reports to us, has been success- 

 fully adopted by Mr. Wales in America. It was originally 

 introduced by Amici, and some rather rough experiments 

 were tried by Mr. Andrew Ross and by Messrs. ISmith and 

 Beck, who came to the conclusion that it was not the best 

 mode of obtaining the desired result; it may, however, be 

 advisable to reconsider this decision. 



Where the largest possible angles of aperture are required 



