COREESPONDENOE. 
217 
amplification, but as to actual definition. Of course, tlie nominal 
magnification can by the use of deep oculars be run up to almost any 
figure ; but what practical worker with high-power lenses does not 
know by experience how useless and deceptive is the further enlarge- 
ment of an object by such means, after that point is reached at 
which, for want of defining power in the lens employed, the image 
begins to lose its sharpness and crispness, and to become indistinct 
and woolly ? 
Mr. Mayall had further noticed that, with Wenham's Reflex 
Illuminator, a high-angled immersion objective gives a luminous field, 
when a pneumo-front gives a dark field ; and he asks. Whence comes 
the luminous field ? In reply, " Crito " makes the singular suggestion, 
that the dry may have had a smaller angle than the immersion lens. 
But what has the angular aperture of the dry lens to do with the 
question ? If the rays were totally reflected from the upper surface 
of the cover, which they would reach after passing through the 
balsam-mounted slide, but beyond which they could not get according 
to the well-known optical law, there could be none to be picked up by 
the dry objective, whatever its angular aperture might be. What 
then could make the field j luminous when the immersion front was 
used, but the entrance into the objective of rays which, with the 
pneumo-front, were totally reflected? But " Crito " asks whether a plain 
glass slip would not answer the same purpose as the Moller's Probe- 
Platte ? No doubt it would, so far as the mere brightness of the field 
goes. But the Probe-Platte serves another purpose. For the objects 
it contains, though attached to the cover and not in contact with the 
glass slip, are brilliantly illuminated, thus proving that the rays 
which enter the object-glass do not arise from mere diffusion of light, 
but that they are bond fide image-forming rays. In the one case the 
image of the diatoms is not there ; in the other it is there. This is, 
as it strikes me, Mr. Mayall' s answer to Mr. Wenham's challenge. 
The pleasantry which " Crito " discharges at Mr. Hogg at the close 
of his letter arises, I think, from a misapprehension of his meaning. 
Certainly he misrepresents his conclusion, which he could not do if he 
rightly understood his abridged line of reasoning. And yet the argu- 
ment, when fully stated, appears to be simple enough. Dr. Parkinson 
proves that chromatic and spherical aberration, which arise from dif- 
ferent causes, are to be corrected by different and independent means. 
Theoretically, therefore, as Dr. Parkinson states, both may be corrected 
together, and a perfect lens obtained. It is equally true from the rea- 
soning that theoretically either of them may be corrected whilst the 
other is left uncorrected. In other words, a lens may be achromatic and 
not aplanaticp or aplanatic and not achromatic. From this the infer- 
ence drawn by Mr. Hogg is, not, as " Crito " incorrectly states it, that 
" some object-glasses which are not achromatic must be aplanatic," 
which is simply absurd, but "that all chromatic aberration does not 
involve spherical aberration," which was the conclusion to be esta- 
blished. 
I am, Sir, your obedient servant, 
Edmund Carr. 
