1 6 transactions of the Society. 
in" resolution is so small as to be hardly worth taking into account. 
What it does effect is the sharpening and clearing of detail already 
resolved, as I have attempted to explain above. 
With all apertures it will be found that blue-green light yields 
the best results. We see, therefore, that blue-green monochromatic 
illumination with low and medium apertures both increases resolution 
and sharpens up the image, but with high apertures it merely 
sharpens the image. 
A great deal has been said with regard to resolution by photo- 
graphic methods, viz. that because photography utilizes rays of a very 
short wave-length, therefore a far greater amount of resolution is 
secured by its use, and that many objects have been discovered by 
photography that were invisible by ordinary vision. I cannot, 
endorse this statement that a greater amount of resolution is possible 
by photographic methods, for the reason stated above, viz. that the 
short blue waves are weakened in passing through the glass ; it is 
light of a longer wave-length that in reality impresses the image on 
the plate. In the writings on this subject a great deal of inexact 
language is made use of, such as “ monochromatic blue light,” for 
when you inquire how was this obtained ? you are told by means of 
an ammonio-sulphate of copper cell. As this cell passes some red and 
green light, it is inaccurate to term the light monochromatic.* In 
such a case it is the green or blue-green light which impresses 
the plate. It is quite out of the question that light as short as 
•4 yL l has any effect at all with wide angles. In practice on the 
most delicate diatom markings I have found scarcely any increase 
of resolution with wide apertures by photographic methods. 
Next, with regard to the discovery of objects by means 
of photography. These discoveries lie among such objects as 
flagella, which are visible solely on account of differences of light and 
shade. Photography accentuates this difference by “ time impression,” 
a case precisely analogous to the photography of very minute stars 
which are quite invisible with the telescope they were photographed 
with. “ Time impression ” means that with a photographic plate the 
effect of a very weak light is cumulative, whereas on the retina a very 
weak light either stimulates the retina or it does not, therefore 
accumulation goes for nothing. 
For example, a very fine jet of water may be issuing from an 
orifice ; a person with not very good sight might not be able to perceive 
the water, but let this tiny stream accumulate in a cistern for a week 
then he would be almost blind if he could not see it. This is the way 
flagella and such like objects are made visible by photography. The 
more unskilled the observer, and the more uncritical the method of 
his work, the more objects will he discover by photographic means. 
* An excellent test for monochromatic light is the examination of the brilliant 
spectrum at the back of an objective of medium power obtained by illuminating a 
coarse diatom such as a Pinnularia with an oblique beam. If an ammonio-sulphate of 
copper cell is interposed green and some red will still be seen, which proves that the 
illumination is not monochromatic. 
