146 NEW BOOKS, WITH SHORT NOTICES. 
Section 8.—104 pages on micro-physics. 
Section 9.—62 pages on micro-chemistry. 
Section 10.—113 pages on the morphology of vegetable or- 
ganisms. 
The purely technical character of this handbook will be at once 
inferred from the brief abstract of contents here given. For even in 
the concluding chapter on vegetable morphology the physico-mathe- 
matical method is as equally predominant as in the rest of the work. 
It is not possible to convey by any condensation or analysis an 
adequate idea of the profound research and practical knowledge 
displayed by the authors in their treatment of the several sections ; 
but it may interest our readers if we touch upon a few points which 
are either new to the majority of English microscopists, or on which 
the judgment of competent experts, whose opinion derives weight 
from their special experience, is opposed to the general belief and 
practice in this country. 
1. Binocular microscopes do not find favour in Germany. A 
stereoscopic arrangement must of course produce the same appearance 
of solid dimensions of objects which other instruments of the kind 
effect ; but whether, say our authors, any aid is thereby afforded to 
scientific observation — for example, whether the discrimination of 
actual differences of form and density is thus facilitated—we must take 
leave to doubt. 
The authors’ explanation of the stereoscopic effect, so far as it 
depends upon physical grounds, is as follows. Each of the two pic- 
tures formed by the two halves of the objective shows an altered 
distribution of light and shadow as compared with the light and shade 
of the picture ordinarily transmitted through the whole area of the 
objective, the shadow prevailing on the side of the excluded half area, 
so that a shift of light and shade in opposite directions in the two 
pictures is effected. Which of the two sides is presented to the right 
or left eye depends on the particular arrangement of prisms, but this 
makes no difference in the causation of the stereoscopic effect. 
How far the depth of field (apparent difference of level of different 
points of the objective image) modifies the stereoscopic effect is again 
matter of contention; no one doubts that a certain depth exists, but it 
is certain that its influence in completing the stereoscopic effect is 
unimportant, and by no means necessary. For just as the superficial 
photograph impressions on the ordinary stereoscopic slide unite in a 
picture giving perspective effects, so must the images of the binocular 
cause an impression of corporeality, even if there were no depth of 
field. Helmholtz explains* the production of stereoscopic effect by 
referring to the position of the dispersion circles caused by points in 
the object lying in front of or behind the focal plane of the objective 
image: that is to say, that depth of field is an essential condition of 
stereoscopic effect. But our authors object that, independently of 
this supposed action of dispersion circles, the pictures formed by the 
two half areas of the objective are already differentiated by altered 
distribution of light and shade, and therefore of themselves bring the 
* ©Handbook of Phys. Optik.’ 
