230 REVIEWS. 
termined without the least knowledge of the position of its stage? 
The fact that the apparent size of the object or image is simply 
a question of angle, and that, if expressed in linear measure, that 
measure must be taken at a uniform distance for all observations 
liable to be compared with each other, — in other words, that to use 
sines and cosines intelligently it is indispensable to assume a uni- 
form radius — would seem to be too simple for argument, were it 
not that a misunderstanding in regard to it is a common and con- 
fusing error. Ten inches having been generally adopted as the 
standard distance of measurement in microscopy, and being a 
the same time a most convenient distance, there seems to be no 
reason for reconsidering the choice; or, for assimilation to the 
metric system, 250 millimetres mer be substituted without any 
considerable error. 
f the minor errors and inadvertencies in this part of the book, 
may be mentioned the following :— The use, as a condenser in stu- 
dents’ microscopes, of the concave mirror transferred to a separate 
stand (it may often be mounted on the stage for the same purpose), 
is mentioned, but without the special commendation which it de- 
serves. It is not stated that Tolles’ students’ microscope can be 
obtained with the usual form of fine adjustment which is to be pre- 
ferred to the “new method” mentioned ; nor that ‘the fine adjust- 
ment on the stage ” of some of the same instruments, and of those 
prepared by some other makers at the present day, is so firm as to 
be reasonably satisfactory, while similarly situated adjustments on 
a lighter class of instruments are usually so insecure as to be en- 
tirely worthless. Following the lead of some of the dealers, the 
fine-adjustment screw is called a “ micrometer screw,” an unfortu- 
nate phraseology which, to the writer’s knowledge, has often proved 
confusing to students. Powell and Lealand’s splendid No. 1 mi- 
croscope is mentioned as the ne plus ultra of instruments, whereas 
it is only fair to admit that the choice between such instruments as 
a ee Ross’ large stands, ete. ) and the best stands of the 
Jackson” model, is a fair field for the use of taste and judgment, 
neither form enjoying a conceded and unqualified superiority. Most 
eye-pieces are said to be composed of achromatic lenses, a mistake 
which is liable to mislead beginners. The student is advised, un- 
der some circumstances, to increase his power by removing the 
field-lens of his eye-piece, a worse than questionable procedure ; nor 
is he reminded to blacken the inside of the pasteboard draw-tube 
RS ie ea le SM tee +> <r 
