CORRESPONDENCE. 
153 
rival opticians, who at once seized upon tiie improvement and imme- 
diately set to work to produce similar lenses. I may remind Mr. Slack 
that the best glasses of this period had only about one-half this 
angular aperture for the same focal length. These are the object-glasses 
of the past, and such as he would now resuscitate, and call improvements. 
I would further remind Mr. Slack of the statement of an authority on 
these matters : that the separating power, i. e. that which constitutes 
the real power of discovery in the ultimate structure of muscular 
fibre, tissue, or cell, increases with or is proportionate to the chord 
of the angle of aperture, at least up to 150°. Has Mr. Slack any proof 
to offer to the contrary ? Is it that he sees P. hippocampus with 
Zeiss's ^th with an angle of 48° ? I am in possession of a ^th by 
Dallmeyer, angular aperture 125°, which exhibits none of the defects 
Mr. Slack dilates upon as pertaining to large-angled objectives. This 
lens has a fair working distance for a thick cover-glass ; it can be 
converted into an immersion lens by m.eans of another front, giving 
about the same angle of aperture ; with the immersion front sufficiently 
unscrewed the aberrations can be balanced for a dry object when the 
angular aperture is reduced to 90° or thereabouts, and I detect no 
chromatic aberration. Thus it appears to me that Mr. Dallmeyer has 
succeeded in placing at our disposal a large-angled objective— for both 
wet and dry objects, and also as a comparatively small-angled glass if that 
be desired — nearer to perfection than any objective I have seen or 
worked with, Nevertheless, I must confess my preference for the 
large-angled glass, because of its precision of focus, the sine qua non, 
as I deem it, of perfect corrections. With this new objective of 125° 
I can see all that I have ever seen with any ith of the same angle, 
and I need not dilate upon the facility of using a ^th as comjoared 
with an ^th. 
As to the assertion that a Beck's aVth, advertised aperture 140°, 
only measures 128°, I would much rather place my faith in the high 
character of the optician and his experience in recording an exact 
measurement than upon the amateur performances of Mr. Slack. 
I do not apprehend anything like astonishment on the part of 
physiologists who may read my letter. I may, however, refer those 
unacquainted with the work of practical histology to the ' Transac- 
tions of the Pathological Society of London,' and extending over a 
period of upwards of twenty-eight years. It is scarcely possible to point 
to more able testimony to scientific microscopy, or to more important 
evidence of the eagerness w^ith which the medical profession has 
availed itself of the latest improvements in high-angled powers, than 
that contained in the twenty-five volumes of this Society. Again, 
turn to Continental work ; take, for example, the splendid volumes of 
Strieker, ' Manual of Human and Comparative Histology.' The main 
results in high-power definition spoken of in these volumes have been 
produced with high-angled lenses. It is scarcely necessary to say that 
the greater part of the valuable pathological work of the schools has 
been done with high-angled objectives ; for what are we to infer when 
such a man as Dr. J. Hughes Bennett, the eminent Professor of Pathology 
in the University of Edinburgh, tells us, in a lecture delivered to the 
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