CORRESPONDENCE. 
211 
On Improvements in Illumination. 
To the Editor of the ‘ Monthly Microscopical Journal.' 
27, Montague Street, Edinburgh, 
August 30, 1875. 
Sir, — Whether the present controversy on angular aperture and 
the rival merits of English and foreign lenses may, or may not, result 
in enlarging the defects while minimizing the excellences of our 
objectives, or whether the differences between the dot-showers and the 
Yalentin-knife men, where the one party looks for glasses having only 
one definite focal plane, and the other for such as have half-a-dozen 
planes at once, are likely to be adjusted by a prize glass to be con- 
structed under the superintendence of the “ R. M. S.,” which shall in 
some mysterious way combine both those idiosyncrasies, is a question 
I care not to meddle with. My purpose here is, not to obtrude 
opinions on others, but rather to seek advice and assistance in a 
matter, the importance of which is incontrovertible. I am alluding 
to improvements in illumination. 
There are, of course, other points in which I should like to see 
improvement, or, at least, the liberty of choice afforded us ; for 
instance, I should like to see some portion of that careful correction, 
which is said to be lavished on our objectives, extended also to eye- 
pieces ; and some contrivance hit upon to enable the diaphragm with 
its iris arrangement, instead of being a fixture, as at present, to be 
smoothly traversed from left to right,* at right angles to the axis of 
the tube, so as to combine oblique illumination with complete exclusion 
of all extraneous rays, and this without adding materially to the 
thickness of the stage. 
But these, I suppose, are things rather to be hoped for than 
expected. 
With regard to illumination, though I have hunted through divers 
works to find what I wanted, my search has hitherto been fruitless. 
They either recommend specialities of limited application, or their ex- 
periments are like those of Mejnour in Bulwer’s ‘ Zanoni,’ — successful, 
it is true, in their own hands, but with the finishing touch concealed. 
Many persons have a strong prejudice against all sources of 
illumination with which glass, — especially quicksilvered glass, — is 
mixed up, as invariably introducing disturbing elements, which it is 
the worker’s business immediately to get rid of. Compare Dr. Pigott’s 
remarks in the ‘ M. M. J.,’ vol. xiii., pp. 152, 177. Therefore I have 
been thinking whether some improvement in this direction might not 
be effected by substituting for our present concave mirror of quick- 
silvered glass, — at least as an optional alternative, — a concave surface 
of some material perfectly white, and yet perfectly free from glistening, 
the surface to be brought to a state of absolute smoothness. As polished 
silver would probably be quite as offensive as our present arrangement, 
it occurred to me that the surface might be covered with white enamel, 
* In the better German microscopes the diaphragm certainly does slide in 
on the right side (cf. ‘Nageli u. Schwendener,’ p. 93); but there is nothing 
answering to the above requirements, — at least I have seen nothing. 
VOL. XIV. 
Q 
