46 : Hed: Clark on the Microscope. 
since assured, by the approval of many. of those who have nae success- 
fully employed the microscope in physio sy investigation 
periority in resolving power possessed by object-glasses of rene an 
aperture is obtained at the expense of other advantages. For even grant- 
adequ 
Jens can only be secured by the greatest etnies in the adjus alison 
of the mirror a r must be su to zi e to the object the 
best possible illumination. If there be any failure in these conditions, the 
performance of a lens of very wide angular aperture is very muc. or 
art 
pressly requ uire this condition, it is a source of great inconvenience and 
loss of time to be obliged to be continually making these adjustments ; 
and a lens, which, when adjusted for a thickness of glass of <3,” will 
without much sensible deterioration with a petro either or 
so or of +45”, is practically the best for all ordinary purpose 
over, a lens of moderate aperture has this very great advantage ‘that tis 
parts of the object which are less perfectly in focus can be much better 
* seen; and therefore that the relation of that which is sae distinctly _ 
discerned, to all the rest of the object, is rendered far more apparent. 
t me remind you, further, that almost all the great achievements of 
orks eee have been made by the Re wgper seit of such ob- 
jectives as I am mmending. There can. be no question about the 
isa proportion of the results which pt | microscopists may claim, 
in nearly all departments of minute anatomical, physiological, botanical, 
or zoological investigations, since the introduction of this invaluable aux- 
iliary ; and it is well known that the great majority of their instruments 
are of extremely simple construction, and that their objectives are gener- 
ally of very moderate angular aperture. Moreover, if we look at the 
h 
Owen, the ‘ Researches into the Structure of Shell’ earried out by Mr. 
Bowerbank and myself, the ‘ Physiological Anatomy’ of Messrs. Todd 
and Bowman, the first volume of the ‘ Histological Catologue’ by Prof. 
Quekett, and "the ‘ British Desmide’ of Mr. Ralfs,—we find sure reason 
to conclude that these researches must have been made with the instru- 
mentality of lenses, which would in thd present day be regarded as of 
ven limited cepa hope that, in these remarks, T shall not be 
w 
