ON HIGH POWERS, AND A STEADY MICROSCOPE. ta 
to a question that I put to the doctor, he told me that he had 
traced points of structure in tissues with the ;th that he 
could not trace with the ~th. Mr. Brooke says emphati- 
cally that a Ith will not show what can be elucidated by a 
4th or ;;th, and I think it may be said also, with equal 
truth, that a th will not show what can be elucidated with 
a=ith. Mr. Brooke concludes this extract with a remark 
well deserved by Dr. Beale, for certainly no one has worked 
at all equal to him with a th. 
Extract IV is evidently from the pen of Mr. Slack, Editor 
of the ‘ Intellectual Observer,’ a deservedly popular periodical, 
having a large circulation. With respect to the impossibility 
of keeping down errors in object-glasses of deeper power than 
_,th I can with confidence affirm, that whatever errors can 
be kept down in a +!,th are equally kept down in a =4th; in 
fact, | have never seen errors so beautifully corrected as they 
are in this unique objective; certainly, in working with it, 
care 1s required, but when accustomed to use it the difficulty 
soon ceases, and you can almost focus for your object with 
the rack and pimion motion. As to not focussing through 
glass but such as will not bear handling, all who have seen 
the circulation im the Valisneria under it must know that 
this is not'true, as the glass covering has continually to be 
taken off and wiped; and I should think Dr. Beale could 
not have worked so constantly and so ably with his ~.th if 
he had not been able to use covering glass that would bear 
handling. As regards my own 4th, I have no object-glass 
in which the corrections are so exact as in it; the flatness of 
the field, the sharpness of definition, the depth of penetra- 
tion, and its power of working through moderately thick 
covering glass without injury to its resolving power, all prove 
the objective to be first-rate, and I feel confident no 1th 
can equal it. In the paper from which Extract IV 1s taken 
there is an allusion to Smith and Beck’s th. I have seen 
the performance of this objective, and it certainly would not 
work through covering glass that my ;;th worked through ; 
and I have stated that the jth works through covering- 
glass almost as thick as the ~,th will work through, and cer- 
tainly through thicker covering than the -;),th will work 
through, and the sharpness of definition between the ~.th 
and the ~:th is most marked. Every one unprejudiced must 
give the preference to the {!;th. 
A few words on the paper of J. J. Plumer, Esq., “On the 
Choice of a Microscope.” It evidently supposes that the 
best instrument can only be obtained of one optician. In 
