CORRESPONDENCE. 
213 
this will be a venial fault with those of kindred tastes, — those, I mean, 
who believe with the poet, 
“ The proper study of mankind is — fungi.” 
I have since endeavoured to know it better. 
It would be superfluous to call it profound ; for all German 
scientific works at least try to be that. It is that, and something 
more. It is a thoroughly practical treatise on applied optics, that is, 
on optics applied specially to the construction and correction of micro- 
scopical instruments, — a regular optician’s vade mecum, with all the 
whys and wherefores reasoned out to the end, — in short, a veritable 
book after Mr. Wenham’s own heart. Indeed, in many passages its 
language is almost identical with some of Mr. Wenham’s recent utter- 
ances ; so that the editors might seem rather to have been translating 
than composing, only that they happened to publish their remarks a 
few years earlier. And throughout the book there is an absence of 
that spirit, so general in German writers, of affecting to ignore all 
that has been written on the subject by other nations. They quote 
Scotch, French, English, American, and Dutch authorities with perfect 
impartiality, thus recognizing that science, like goodness, is the pro- 
perty of no particular nationality. In their chapter, “ How to deter- 
mine Angular Aperture,” after discussing the various methods proposed, 
and Mr. Wenham’s amongst them, they remark, “ This [Wenham’s] 
method has indisputably the great advantage, that we are enabled by 
it to determine, not only the aperture of the objective in respect of 
the whole amount of light it admits, but also the really available part 
of it, that is, the part which supplies sharp and correct images.” 
See p. 168. 
I may add, that the work is written by men of acknowledged 
eminence as mathematicians, who are at the same time notable micro- 
scopists, and of high repute for their microscopical researches in their 
own particular line ; so that their statements w r ill hardly be open to 
the sarcasms that might be levelled at microscopical assertions by 
writers on optics ignorant of microscopy, or at optical remarks by 
microscopists careless of optics. 
I do not mean that all their theories will be acceptable to our 
London opticians. The following, for instance, will, I know, be very 
unpalatable : “ The use of condensers is in most cases superfluous, 
where the mirror is sufficiently large, and can be brought up near 
enough. The use of such things has a meaning only where one pur- 
poses to enlarge the apertiu'e of the incident cone of light.” . . . 
“ Condensers, therefore, are efficacious only in two directions ; they 
give to the cone of light, which illuminates a particular area of the 
field of view, an equal intensity in its entire cross-section ; and, in the 
nest place, enlarge its angle of aperture. As for the other assertions 
regarding the effects of condensers, that they dissipate the interference 
lines at the margin of the object, and resolve difficult details pro- 
portionately better, the more completely the correction of their 
aberrations has been carried out, that is pure imagination.” See 
pp. 91, 255. This, of course, is rank heresy ; but I suspect that 
Q 2 
