PROCEEDINGS OF THE SOCIETY. 
439 
negative and on the print be cumulative. Mr. Nelson considered it 
highly important to dispose of the lateral development theory, for if such 
an error were allowed currency without challenge, it would very soon be 
said that a photomicrograph, on account of this lateral development, had 
no scientific value. 
Mr. Nelson also read a short paper “ On the Use of Monochromatic 
Light in Microscopy,” and exhibited and described the model of a new 
and simple apparatus for obtaining the same by means of a glass 
prism. 
Mr. Mayall said he recollected the original apparatus designed by 
the late M. Prazmowski at the time he was in partnership with Hart- 
nack ; it was troublesome to manage, so that not very much was done 
with it. That made by Zeiss, later on, was practically the same thing. 
He thought, however, that the apparatus before them was very likely to 
do good work, and the facility with which the prism could be turned, 
and the illumination varied from one end to the other of the spectrum, 
at once commended it to notice. He understood Mr. Nelson to say that 
by changing the monochromatic light from yellow to blue, the resolving 
power of the objective could be plainly seen to be augmented by an 
amount equivalent to *1 N.A., and that in many cases an ordinary 
achromatic objective produced images as perfect as those given by 
apochromatic objectives. These were points of special scientific interest. 
He should not be satisfied, however, until Mr. Comber had seen it and 
put it through its paces, using sunlight, because now they were not 
content with mere performance with the Microscope, but they must have 
good photographic results as well. Of course, to be of permanent value, 
the apparatus must not be made of wood, as in the model. There would 
be no difficulty in adapting an inexpensive form of spectroscope for the 
purpose. 
Mr. Nelson also described a new Projection Microscope fitted with 
a special condenser made of three flint lenses so as to embrace the whole 
cone of 82°. The only novelty about it was the system of collecting 
the light, by which a beam of 4^ in. was brought down to one of 1^ in., 
and by passing through the two lenses placed in a water-trough, a beam 
of parallel rays of great intensity was obtained for use in projecting the 
image upon the screen. It was necessary to have a different condenser 
for each objective, as the one must be perfectly adapted for use with the 
other. He had at present only two, one being for use with Zeiss’s A A, 
equal to about an ordinary 1 in., and the other being a lower power. A 
number of slides were exhibited upon the screen. 
The President said he had been very much struck with the beauty of 
the views which had been shown, and thought that it would be a great 
acquisition to any one who wanted to give exhibitions of microscopical 
objects. Very few schools should be unprovided with such an apparatus. 
He was sure those present felt greatly obliged to Mr. Nelson for what 
he had shown them. 
Mr. Mayall said that some seven or eight years ago, when Dr. Hugo 
Schroder first came to England, he gave a description of a Microscope 
for projection purposes which he had devised, examples of which were 
