168 Transactions of the Society. 



The 2 in. and 1 in. are good working lenses. The £ in., is prob- 

 ably one of the very finest old objectives ever made. When used 

 with Gifford's screen it will stand a 40 eyepiece and a full illumi- 

 nating cone without breaking down, and thus arranged it exhibits 

 clean, well-contrasted pictures of various sensitive objects. With 

 the screen and a solid cone of 0*35 N. A., it will completely and 

 cleanly resolve Grayson's 30,000 band. This band is very nearly 

 at the practical working limit of a good apochromat of similar 

 aperture. The \ and \ in. will both easily stand the full cone of 

 the old condenser, and with it will clearly dot P. angulation and 

 afford perfectly critical images. The ^g in. has, of course, a very 

 low optical index, but its corrections are remarkably fine, and 

 under it P. angulatum shows a strongly contrasted structure, the 

 "dots" appearing so conspicuously black, round, and clearly ren- 

 dered that one marvels how the "chequered" resolution could have 

 been until so lately considered the proper image, when it is thus 

 evident that lenses and condensers were constructed in 1850, quite 

 as capable of truly picturing this diatom as any dry objectives 

 made to-day. It is manifest that not the instruments, but the men 

 at the eye end were to blame for the uncritical results and the 

 erroneous interpretation of what they saw. 



There is another point worthy of notice regarding these old 

 objectives constructed in the very infancy of the modern 

 Microscope, i.e. when the instrument was first furnished with a 

 properly constructed achromatic substage condenser, capable of 

 affording a sufficiently large solid cone to yield truly critical 

 pictures with the highest apertures then available. This is the 

 cleanness of the critical images given by the £, \, \, and ^ i n> 

 which will bear moderately deep eye-pieces without breaking up 

 or appearing " rotten." As a matter of fact, they were con- 

 structed with a view to standing fairly high oculars, the eye-pieces 

 supplied with them magnifying respectively 5, 10 "4, and 20*2, 

 the 10 - 4: ocular being probably intended for general working pur- 

 poses. We therefore find that the eye-piece power at present 

 ordinarily employed is about the same as that used on first-class 

 instruments nearly sixty years ago, while the 20 ocular would be 

 considered high by many modern microscopists provided with 

 apochromatic objectives. A modern objective of 0-91 N.A., 

 which with a large cone would bear eye-piecing up to 1850 

 diameters without exhibiting a fuzzy or pale image, could not be 

 reckoned as anything but good, yet the old nominal y 1 ^ in., with 

 the 10 eye-piece, affords a power equal to the above, and the 

 details of objects appear sharp and well contrasted. Of course, it 

 is much easier to obtain good correction with a low than with a 

 high optical index, and it may be safely asserted that sixty years 

 ago no objective of • 9 N.A. of much less than 90 I.M.P. in a 

 10-in. tube could have been sufficiently well corrected to produce 



