WOODWARD, ON MONOCHROMATIC ILLUMINATION, 255 
closed the end of the blackened tube. The object of this 
lens was to increase the dispersion, and so to increase the 
time of exposure necessary to produce a decided impression 
on the sensitive plate. At two feet from the lens was the 
plate-holder. In front of the sensitive plate was a slider 
with an aperture, so arranged that two small areas of the 
plate could be exposed successively. The field being evenly 
illuminated, one of the areas was exposed for twenty-five 
seconds, when the plate was covered ; an assistant removed 
the prism, slid in the ammonio-sulphate cell, and altered the 
position of the mirror, so as to throw the sunlight through 
the cell upon the concaye amplifier as before. An even 
illumination having been obtained, the second area in the 
sensitive plate was exposed twenty-five seconds. On develop- 
ment it was found that the part of the plate illuminated by 
the ammonio-sulphate was several times blacker than the 
part illuminated with the prism. Using this sensitive plate 
as a negative, I obtained the print which I enclose, in which, 
of course, the lightest area corresponds to the darkest area 
in the original plate. My reason for using the amplifier to 
disperse the rays in both instances was that the exposure 
must otherwise have been instantaneous, and the slightest 
variation in time would have vitiated the results. 
From a careful comparison of the two modes of illumina- 
tion, and from the photographic experiment, [ am compelled 
to conclude that the ammonio-sulphate cell offers greater 
practical advantages for the purposes of photo-micrography 
than the prism, the small quantity of the other rays which 
are transmitted by the ammonio-sulphate not interfering in 
the least with the results. 
As, however, Count Castracane may be more skilful in 
his manipulation of the prism than I have been, I herewith 
transmit a photograph of Pl. angulatum, taken by myself 
with the Wales 1th and amplifier, magnified 2544 diameters ; 
one by my friend Dr. Curtis, by the same lens, with the 
same power; one by the latter with the Powell and Lealand’s 
4th, magnified 2544 diameters; and enlargements of the 
two latter to 19,050 diameters. These photographs I beg 
you to transmit to Count Castracane, whose address I do not 
know (though I have endeavoured to obtain it through Dr. 
Maddox), with assurances of my highest consideration, and 
with the request that he will send me paper proofs of his 
own best photographs of the same object as obtained by the 
prism. 
By so doing you will confer a lasting obligation upon one 
who is anxious only to get at the truth in this interesting 
optical question, 
