REVIEWS. 229 
trust entirely to the honesty of a responsible dealer or to the re- 
spectability of the maker of the lenses required. The “‘ testing” 
of first-class lenses, by a person unfamiliar with their use, is sim- 
ply a delusion. 
The binocular microscope certainly merits more than the half 
page of careless notice given to it. Although its chief use, as yet, 
is in other departments of study, yet its real value to the phy- 
sician is not even hinted by the passage referred to. Wenham’s bi- 
nocular, the one in actual use, is almost indispensable in studying 
opaque injections, vegetable and animal parasites, etc. At the Troy 
meeting of the American Association last August, the writer showed 
specimens of animal and vegetable tissues (such as transparent 
injections, etc.) under this arrangement with good definition, -an 
even light over both fields, and the stereoscopic effect vivid and 
useful though perhaps somewhat exaggerated, under the ordinary 
first-class objectives as high as one-fifth inch focus, and up to 120° 
ang.ap. Probably, however, satisfactory substitutes for Mr. Wen- 
ham’s arrangement will be in general use for all powers above one- 
half inch at no distant day. 
In giving directions for estimating the magnifying powers, and 
the amplification of camera-drawings of objects, the author fol- 
lows respectable authorities into an error of grave practical con- 
sequence. To measure the magnified image at the exact distance 
of the object is plausibly borrowed from the usage of the telescope 
where the object is at a fixed distance — its apparent size being al- 
‘ways its angular extent at that distance, and the image being meas- 
ured by the object itself, of course at the same distanee. But 
the microscopic object is at no necessary and invariable distance 
from the eye, and its apparent size, 7. e., its angular extent, varies 
with that distance, and is with no more propriety used as a standard 
at the distance of the stage, than at any other distance. Would 
any person measure the image in a simple microscope at the exact 
distance of the object on the stage? or if, for any purpose, a com- 
pound microscope six feet long were constructed, would its image 
be measured according to the same rule? Or, of two dissimilar 
microscopes containing similar objects, say Maltwood finders, the 
stages being concealed so as to be at an unknown distance, would 
any microscopist hesitate an instant before deciding which instru- 
ment magnified the most, such decision depending, Gf course; on 
the certainty with which the magnifying power of each could be de- 
