244 
PROCEEDINGS OF THE SOCIETY. 
This screen, as they were aware, had the F line for the centre of its 
band, and it was found, from the experiments which were carried out by 
Mr. Gifford and himself, that it was not possible to go higher up the 
spectrum into the blue without incurring a loss visually, although 
photographically something was gained. Now, if they could only com- 
bine the Gifford screen with this new method by making the peripheral 
portion a blue whose spectrum began at F, and the centre a green in 
which the spectrum ended at F, he believed an advantage would be 
secured. Some words of caution were however necessary about the 
selection of the colours ; for if they chose two that were widely separated 
in the spectrum, they might exaggerate the effects of the residual chro- 
matic aberrations in the objective they employed ; this of course applied 
with more force to semi-apochromatics than to apochromats. Suppose, 
for instance, that they used a deep red for the centre and a blue for 
the periphery, with a semi-apochromat, it was more than likely that that 
objective would have a sensibly shorter focus for the blue rays than for 
the red ; then if their object should be a diatom having fine structure 
above a coarse, it was not improbable that the image, under these cir- 
cumstances, might be reversed, and the fine structure appear by focal 
adjustment to be beneath the coarse. 
In photomicrography, Mr. Rheinberg’s method would prove useful, 
for there were certain objects having different parts that required dif- 
ferent exposures ; some diatoms formed an excellent example of this, 
for it sometimes happened that a strong eye-spot would be so brightly 
illuminated as to quite blot out a finely perforated membrane lying 
above it ; that is to say, the eye-spot would require a very short exposure, 
while the delicate membrane above it required a longer one. Now, with 
this method they were able to impart a yellow or orange tinge to the 
eye-spot, and so equalise the exposure throughout. He hoped that 
Mr. Rheinberg would continue his valuable investigations, and, now that 
he was a Fellow of their Society, communicate any further discoveries 
he might make at some future meeting. 
The thanks of the Meeting were then, upon the motion of the 
President, cordially voted to Mr. Rheinberg, and to those opticians 
who had so kindly provided the Microscopes for the purpose of the 
exhibition. 
The President announced that at the next Meeting Mr. Lewis Wright 
would give them an exhibition of his Lantern Microscope. 
The following Instruments, Objects, &c., were exhibited;— 
The Society: — Microscope by Jas. Smith. 
The President : — Powell’s Iron Microscope. 
Messrs. R. and J. Beck : — New Reversible Compressorium, and 
coloured sectional diagram of the same. 
Mr. C. L. Curties: — Photomicrographs by Mr. W. C. Rowden. 
Messrs. Watson and Sons : — New Model of the Van Heurck Micro- 
scope ; New Cover-glass Clip for making blood-films. 
Mr. J. Rheinberg: — List of Objects shown by Multiple Coloured 
Illumination : — 
