146 
Transactions of the Society. 
Having described the violet screen I will now show you a photo- 
graph of the spectrum taken through it on a bromide film, such as is 
used for ordinary lantern-slide work (plate I. fig. 3). 
Immediately below is the spectrum taken on such a plate without 
any screen (plate I. fig. 4). 
You will see that if a line be drawn through the centre of both, it 
will occupy very nearly the same position. And if lines be drawn 
representing the mean of average intensity, they will be still more 
nearly in the same relative position. 
The deduction is sufficiently obvious. We can focus our micro- 
objective or astro-telescope or photographic lens with such a screen, 
and then remove the screen altogether for the exposure, and this has 
been borne out in practice. 
But the difficulty comes in here. You have before you a photo- 
graph of the Podura scale, taken in this way with a Powell and 
Lealand semiapochromatic of 1*5 N.A. (plate I. fig. 5) and another 
(plate I. fig, 6) similarly taken with the malachite-green screen. 
These are very poor photographs of Podura, and I have only chosen 
them because both are in the same focus, and both were taken with 
the full available aperture (about *95). The one taken with the 
violet screen is very “ fuzzy,” but the other is fairly sharp. 
The question of using the extreme violet for purposes of photo- 
micrography has already been taken up by Mr. Nelson, and I believe 
I may use his authority for saying that the lenses he used he found 
so imperfectly corrected for spherical aberration as to be useless. I 
also have worked through a number of lenses, including apochro- 
matics, in the hope of finding one properly corrected for it, but have 
entirely failed to find such a lens. 
I am well aware that opticians will tell me that first-rate results 
may now be obtained without the use of any screen, but my experi- 
ence has been that an apochromat with the malachite-green screen, 
which takes most lenses at their best, carries results much further 
than an apochromat without a screen. I would ask whether some 
one is not prepared to construct a lens in which everything has been 
sacrificed to spherical correction for the violet. If this were done we 
should combine the good points of a screen with the advantages of 
working without one, making use of rays of great resolving power on 
the one hand, and photographing with considerable rapidity on the 
other. 
But the screen may even now be useful. By diluting the solution 
somewhat, the band widens out towards line F, and finally overlaps 
it. It is now, of course, not so monochromatic, and we should 
suppose would give inferior results. But we find that the spectrum 
obtained with the diluted solution very nearly coincides with that 
impressed on a bromo-iodide (ordinary) photographic plate without 
a screen, and takes in a portion of the spectrum covered by the 
malachite-green screen, for which lenses, generally speaking, are more 
spherically correct. 
