ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
429 
optics there is nothing in any sense new, and there is neither in this 
part nor the subsequent pages of the book any effort made to give any 
mathematical expression to the complex modern theories of microscopic 
vision. This will not diminish the value of the book to the less 
ardent student ; but it will leave a proportion of those who desire to 
understand fully the principles of modern theory and practice con- 
cerning the Microscope still unsatisfied ; of course the one might have 
been accomplished without detriment to the other. “ La theorie de la 
vision microscopique de M. le Prof. E. Abbe ” is given clearly, but 
scarcely as exhaustively as we are inclined to believe its importance 
at this time demands. At length the theory of Abbe has received 
the careful consideration its value demands ; but this has also brought 
with it a criticism which cannot be ignored, and the value of which 
will only be seen after an equally careful consideration. Like all 
similar criticism it will only lead to what all earnestly desire, a more 
radical analysis, and a firmer establishment and more useful applica- 
tion of what will remain unshaken. But it is all the more needful 
that those concerned in the controversy should have before them, in 
as clear and exhaustive a form as possible, the details of the great 
modern theory of microscopic vision. 
In dealing with the “ general consequences ” of the theory there 
is certainly a fearlessness and an assumption of the inevitable in 
regard to these “ consequences,” which practically conclude that they 
may in their smallest details be placed side by side with the demon- 
strated laws of light. We much prefer an open mind, and although we 
believe that the Abbe theory in the main must remain unshaken, Dr. 
Abbe has himself shown that there are points in detail which have 
needed modification. 
In the use of the more powerful object-glasses Dr. Van Heurck has 
shown himself singularly competent, especially in regard to photo- 
micrography ; hence we are somewhat disappointed to find so limited a 
range of “ test- objects ” considered, especially as those he directs us to 
employ have been for so long in use. What we now need is objects 
that will specifically differentiate the qualities of the apochromatic 
objectives from their achromatic predecessors, especially when it is con- 
sidered that for lined (or “ dotted ”) objects the table so long and use- 
fully printed at the back of this Journal gives the theoretical power of 
any lens. 
We note that the pygidium of the flea is given with much com- 
mendation as a “ test.” No doubt it has its value for this purpose ; but 
with the recent (apochromatic) objectives there is more discoverable 
than was taken as evidence of “ test structure ” a few years ago. The 
minute hair-like bodies covering the surface, and especially their mode 
of insertion, are among these. 
Of course we have a slight reference only to the new object glass by 
Zeiss of N.A. 1’60, and Dr. Van Heurck’s remarkable photomicrographs 
with it are reproduced. In regard to these it must be remarked that 
they can hardly be counted satisfactory, taken as they were with a con- 
denser whose aberrations are of the most emphasized nature, and with 
no attempt at their correction. 
The disqualifications of this remarkable objective are not referred 
