ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
517 
as for crisp definition over an entire perfectly flat field they are unsur- 
passed; for high amplifications the new apochromatic oil 1/12 Zeiss is 
superb. 
The question, whether to receive the image directly from the objec- 
tive on the plate, or to employ some means to project the image, has 
received of late much attention. While fully appreciating the theo- 
retical objections to the direct image, I confess that for low and medium 
powers I continue to use it by preference, as the photographs so obtained 
fully equal in every respect any which I have ever seen made by the 
indirect mode. With high amplifications (1000 diameters and over), 
the conditions are greatly changed by the approach to the limit both of 
the shortness of the focus of the objective and of the length of camera 
which can be advantageously used ; my experience leads me to adopt the 
1/12 in. objective as the one, and not over four feet as the other limit, 
since any given high amplification, say 2000 diameters, can be more 
satisfactorily and more conveniently obtained with a superior 1/12 in. 
connection with suitable optical means to increase the initial magnifying 
power of the objective than with an unaided 1/25 lens and the plate 
removed to a great distance. Until quite recently the various amplifiers 
offered the best means of increasing the power of an objective, but the 
introduction of the “ projection-oculars ” of Zeiss has given us an acces- 
sory for this purpose far superior to the older devices. These projection- 
oculars resemble the ordinary microscopical oculars, or eye-pieces, only in 
general form and in name, being optically a projection-objective in con- 
nection with a collecting lens. The new oil-immersion apochromatic 
lenses, in combination with these projection-oculars, form, undoubtedly, 
the most efficient equipment for high-power work, and have but one draw- 
back — their cost. It is, unfortunately, as true for high-power photography 
as for microscopical observation in general, that the best results are to 
be obtained only with fine, and necessarily expensive, optical appliances. 
If for the satisfactory study of the intimate structure of a cell, or of a 
micro-organism, the most improved immersion lenses are necessary, it is 
to be expected that for the successful photographing of the same, tools 
at least as good are needed. The complicated mechanical arrangement 
for controlling the focusing adjustments from a distance, may usually be 
replaced with advantage by the simple contrivance of cords and weights, 
devised by the writer more than a dozen years ago, which has been so 
generally adopted in this country ; during the extended continued use 
of this little device, it has never been found wanting, responding per- 
fectly to the severe demands of the highest amplifications. A modifica- 
tion for the coarse-adjustment, having pulleys and very heavy weights, 
serves equally well when very low (2 to 5 in.) lenses are used. A 
very stiff spring in the fine-adjustment may sometimes require increased 
friction to prevent the cord from slipping, the necessary traction being 
obtained by heavier weights, or by taking an extra turn of the cord 
about the milled head of the micrometer screw. 
My conclusions regarding the second of the necessary conditions — 
good illumination — are briefly stated; after many experiments with 
various kinds of artificial illumination, and after the examination of 
innumerable specimens of the best work of acknowledged experts, while, 
of course, admitting that good photographs can be made, under suitable 
