174 
Transactions of the Society. 
It occurred to me that a trough containing solution of iron per- 
chloride used as a screen would cut off all the blue end of the spectrum, 
and some of the green, leaving only the green-yellow, yellow, and red ; 
but on the other hand I found that erythrosine plates were only 
sensitive as far down as the yellow, being more especially sensitive to 
this green-yellow. I found that in this way the part of the spectrum 
used for photographing could be reduced to a narrow hand about 
midway between the D lines and the E line in the solar spectrum. 
By this means I succeeded in obtaining the photographs now before 
you, which I have carefully avoided touching up in any way what- 
ever. All that I claim for them is that they show approximately what 
I saw with the sodium flame, and which I was unable to photograph. 
For, by experiments with the sodium flame and low powers, I found 
that to photograph A. pellucida under the powers I had, and with 
either the erythrosine or cyanine plates I had prepared, an exposure 
of from 60 to 70 hours would be necessary. 
Whether these markings are true or spurious, is a question I do 
not touch, but I may perhaps venture to say that they appear to me 
to have as good a claim as those on Surirella. But I think it more 
probable that in both cases they are simply multiplied images of the 
midrib and sides produced by the higher orders of diffraction spectra. 
As already mentioned, it is only at the side of the field of view 
that the second set of lines comes strongly into view. In the centre 
one only gets “ samples,” but now and then there is even there a 
distinct beaded appearance. 
The mounts of A. pellucida used were of realgar or rather 
a higher sulphide of arsenic prepared by melting sulphur with 
realgar. I have found that it is possible, though extremely difficult, 
to make mounts with such a large proportion of realgar, that they are 
of a deep orange colour inclining to red, and that with such mounts 
the coloured screen may be dispensed with. But these extremely 
highly refracting media very soon crack off from the cover-glass like 
phosphorus, which I have been obliged to abandon. 
One of the advantages of working with this arrangement of light 
and a suitable colour-correct plate is that ordinary achromatic object- 
glasses perform almost as well as apochromatics, the visual rays and 
those used for the photograph being practically identical. 
I may add that I have been unable to see these beaded markings 
with any glass of less aperture than 1 • 4 ; the best results were obtained 
by a 1/10 in. of 1’5 and a 1/20 in. of 1*5, both by Powell and 
Lealand, but it was shown exceedingly well by a 1/12 in. of 1 ’43 by 
the same makers. Of these the 1/10 in. is an apochromatic, the 
others achromatics. I have as yet not had an opportunity of trying 
the Zeiss apochromatics of large aperture. I attempted at first to 
photograph without an eye-piece, but once having put in a Zeiss 
projection ocular, results were so much improved that it was used 
throughout. 
