ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 373 
method by which the higher qualities of lenses could be brought out, 
until we were recently provided with an apochromatic form of this 
instrument by the firm of Powell and Lealand. 
But it was not until 1873 that even Abbe saw the need of the pro- 
vision of a condenser ; and this arose from the fact that critical images 
became a necessity by the inauguration of the science of bacteriology. 
But the condenser as he thus for the first time made it was simply a 
chromatic one : by no means of value where the objectives were of the 
highest order and corrected on the best modern principles. Hence it 
became slowly manifest even in Germany, where the condenser had for 
so long been deemed only a toy, that an achromatic condenser must be 
constructed, and we are indebted to Abbe for the introduction, in 1888, 
of a practical form of this. But there had existed in England for many 
years several forms equally simple, and decidedly more suited to 
English stands. 
But with what slowness the necessity for bringing out the higher 
possibilities of the finest recent objectives comes to the German maker 
may be seen by the fact that the firm of Zeiss have, as is so well known, 
constructed an objective beautifully corrected, on the apochromatic 
principle, having a N.A. of 1*63. And for the illumination of this they 
provide a chromatic condenser — a condenser wholly without corrections 
— -having an aperture equal to the object-glass ; so that unless we use an 
absolutely achromatic source of light, we introduce the most vicious 
aberrations. But in the treatise before us the substage condenser is 
taken for granted. It has no history ; it goes with the German Micro- 
scope, which the student is directed to use, as naturally as he is to use 
a knife with a microtome. 
There is much on the theory and practice of illuminating that is of 
value, and the mechanical stage, for so many years looked upon as one of 
the unnecessary luxuries of the English Microscope, is carefully de- 
scribed, as made by Reichert, who follows Zeiss in the practical adoption 
of the English model. 
The micro-camera apparatus of Winkel, Thoma, and Edinger are 
introduced with profit, as we know that such apparatus for drawing is of 
great service to the student ; and the section on the use of the polariscope 
is specially clear and practical. 
There is a chapter on Photomicrography ; and the remainder 
(109 pp. out of 321) is occupied with a description of the apparatus for 
and methods of preparing, cutting and mounting, for study and preserva- 
tion, the material and objects employed for demonstration and research, 
which covers in the usual way the requirements of the student. 
Lens Work for Amateurs.* — A workshop receipt-book by a workman 
who evidently understands nothing of the theory of optical instruments. 
In one part of this work there is a pretence of much theoretical know- 
ledge, but a comparison with the English translation of ‘ Naegeli and 
Schwendener on the Microscope ’ reveals that both the text and figures 
have been abstracted wholesale. 
When the ipsissima verba of Naegeli and Schwendener’s text are left 
for purposes of condensation, the blunders made by the author show 
* H. Orford, ‘ Lens Work for Amateurs,’ London, 1891 flS95\ 8vo xv. and 
231 pp., illust. 
