96 
CORRESPONDENCE. 
the angles of aperture is not explained and certainly not proved to 
my mind by anything said or quoted by Mr. Slack. 
In vol. XV. of ' Les Mondes ' (p. 482 et seq.) Dr. Hartnack dis- 
cusses the conditions under which the highest definition is obtainable, 
and he lays great stress on the importance of the utmost limit of 
angular aperture in high-power immersion objectives. Trying Zeiss's 
Nos. 1, 2 and 3 immersions ( = ^th, -i^th, and ^^th) by the test of 
deep oculars, I find Dr. Hartnack's observations borne out ; the image 
with these comparatively low-angled objectives breaks up with any 
magnification beyond about 1000 diameters. On the other hand I 
have found similar powers by Hartnack and by Powell and Lealand, 
with their greater apertures, give far greater amplification, still retain- 
ing fine definition. 
Taking Mr. Slack's summary of his results : — He says that 
" opticians have been encouraged to make excessive apertures substi- 
tutes for good corrections ;" if this means anything, I take it to mean 
that opticians have been encouraged to make lenses that give bad 
images ? In other phrase, the demand has been for lenses of inferior 
rather than superior quality ? By way of absolute negation of this 
dictum I point to his own criticism of Powell and Lealand's new 
ith, in which aperture has been carried to the highest limit con- 
sistent with the accuracy of corrections for which these opticians are 
renowned. Secondly, he says that " naturalists and physiologists have 
been too contented with feeble resolving powers." If he had said that 
many English opticians have been too contented with mediocre lenses, 
that they slumbered over the introduction of improvements in high 
powers until both on the Continent and in America lenses were pro- 
duced that have fairly given microscopy a new start, then he would 
have stated an important, though unpalatable, truth : as it is, the 
naturalists and physiologists have been rapidly providing themselves 
with high powers from Hartnack and others, so that the old objectives 
that were formerly in use in the medical schools are now looked upon 
as the stepping-stones to higher and more difficult investigations, 
such as require the aid of more powerful objectives. Thirdly, he 
speaks specially of the utility of the Keflex Illuminator. The Eeflex 
Illuminator is a very ingenious contrivance by which new and curious 
effects of oblique illumination can be produced, both by reflexion and 
refraction ; Dr. Woodward's photograph of Podura is the finest 
example I have seen of one of its uses. Mr. Wenham has hitherto 
seemed not to have observed that his Eeflex Illuminator furnishes a 
most elegant practical refutation of his own position with reference to 
the " aperture " question. He has challenged " anyone, to get, through 
the object-glass with the immersion front .... any portion of the 
.... rays " that would " be totally reflected " with a pneumo-front 
(vide ' M. M. J.,' No. xxvii., p. 118). If he will try the experiment 
on Moller's Probe-Platte with his Reflex Illuminator and a high- 
angled immersion lens, he will see a luminous field ; whereas, with 
a pncumo-lens he obtains a dark field. Whence comes the luminous 
field ill tha immersion lens if not from its having the power to collect 
rays which are totally reflected when the pneumo-lens is used ? 
