396 
Transactions of the Society, 
(a) With dark -ground illuminator alone, gave an intensity 1. 
(5) With dark-ground illuminator and bulls’-eye focussed to 
give parallel light, intensity 3 • 45. 
( c ) With dark-ground illuminator and bull’s-eye focussed to 
form image 2£ in. from illuminator, intensity 15. 
- Experiment 2. — A specimen of Ooscinodiscus mounted in an 
aqueous solution — 
(a) With dark-ground illuminator alone, intensity 1. 
(b) With dark-ground illuminator and bull’s-eye focussed to 
give parallel light, intensity 3. 
(c) With dark-ground illuminator and bull’s-eye focussed to 
give image 2 { in. from illuminator, intensity 7. 
The increase in intensity is very marked and requires explana- 
tion. 
The question as to whether this illuminator delivered a larger 
angular cone upon the object under the different conditions was 
considered when the image of the light was focussed to a distance 
only 2 ^ in. from the illuminator. It was still about 30 times the 
focal distance away, the light from it was not far from being 
parallel compared with the reflecting power of the curves, and it 
should not affect the optical effect of the apparatus. Careful 
measurement showed no appreciable increase in the angular size 
of the cone of light delivered upon the object. 
The nature of the image of the illuminant formed by this 
illuminator did not give any serious distortion. 
The question as to whether such an increase in light could be 
occasioned by reflection from surfaces of the slide, cover glass, 
object glass, or the object itself, seemed improbable on account of 
the large amount of such increase, but the following experiments 
appear to show that such is the case : — 
Experiment 3. — An isolated malaria parasite which filled a red 
corpuscle, stained and mounted in balsam, was illuminated by a 
dark-ground illuminator, and the light intensity reduced by 
means of a pair of neutral glass wedges until the object was only 
just visible. The use of a bull’s-eye exactly as in Experiments 1 
and 2 did not render it more visible. 
Experiment 4. — A stained anthrax bacillus, mounted in balsam, 
gave the same result. 
Experiment 5. — A specimen of stained blood, mounted in 
balsam, viewed with a low power and put slightly out of focus so 
as to give a uniform matt surface of light, also gave the same 
result as Experiment 3. 
Experiment 6. — A stained section of tissue, mounted in balsam, 
gave an appreciable increase in brilliancy when the bull’s-eye was 
used, but not as great as in Experiments 1 or 2. 
It appeared, therefore, that specimens mounted in balsam, 
