36 
CORRESPONDENCE. 
Dr. Carpenter has advocated, for many years, small-angled objec- 
tives "for tlie ordinary purposes of scientific investigation," and 
" .... for almost every purpose -except the resolution of diatom- 
tests, objectives of moderate angular aperture are to be decidedly pre- 
ferred." In bis fifth edition of ' The Microscope ' (published with 
the assistance of Mr. Slack), he says, page 208, " several opticians 
now make objectives of these limited apertures inches of 40° and 
iths of 75°), of excellent quality, and very moderate price," and page 
819 — "the excellent small-angled ith and ^ih made by Mr. Swift 
expressly with this view." * 
Messrs. E. and J. Beck supply their y\ths and ^ths with both 
small and large angular apertures, besides the objectives of their 
" Popular " and " Universal " series, in both of which the angular 
apertures are small. 
Messrs. Powell and Lealand supply ^ inches with angular aper- 
tures of 40° or 70° ; Jths of 95°, 130°, or 145° ; iths of 140° ; but I 
am informed they will supply ^ths with an angular aperture of 100° 
if desired. 
Messrs. Eoss and Co. supply their patent i inches, y o^ths, and iths, 
and will supply most likely, " later on," their iths, with both large 
and small angular apertures. 
This clearly proves what Mr. Crisp said at this month's meeting 
of the Eoyal Microscopical Society: " Opticians are not to be blamed 
for making their objectives with large angles. The blame entirely 
rests with microscopists, who want to be educated. . . . Objectives 
are made with large angles to meet the demand." 
If the above facts are taken into consideration, and also that the 
leading English opticians will, no doubt, produce objectives of any 
focal length and angular aperture desired, I do not see the reason 
why foreign objectives should be brought forward in such a prominent 
manner to the detriment of English ones. The cheapness of some of 
the foreign objectives weighs, certainly, very much in their favour ; 
but to balance this there is the great uncertainty as to vjJiat quality 
of objectives one may get when ordering them, if the opportunity of 
selection is not afforded. The best foreign objectives are only obtain- 
able under the most favourable circumstances. 
I have received some very good objectives from one Continental 
optician, but have also been very glad to dispose of objectives received 
from two other Continental opticians as soon as I had examined their 
performances, which were certainly much less than " moderately bad " ! ! 
I do not believe that the most strenuous advocate of foreign 
objectives is prepared to maintain their superiority, or even their 
equality, to the objectives produced by the three leading firms of 
London opticians. 
I am, Sir, your obedient servant, 
A. DE SOUZA GUIMARAENS. 
* Vide also Mr. Slack's letter, ' M. M. J.,' vol. xi., p. 264, June, 1874, &c. 
