PROCEEDINGS OP THE SOCIETY. 
419 
seen that it worked parallel. It was made long in order to get it over 
the stage. He had not secured the design in any way, but left it entirely 
open to any one to adopt it if they wished to do so. 
Dr. W. H. Dallinger thought this form of compressorium certainly 
had the advantage of being able to be turned over completely and easily. 
Beck’s form had also the advantage of being parallel, but on account of 
its sliding movement it was very apt to injure minute organisms placed 
within it. The one now exhibited had a parallel motion, but it was 
only so when it could be used in a parallel way ; he was afraid, however, 
that it might be found a little hurtful for high-power work. It was, 
apart from this, an exceedingly simple and useful device. 
Dr. G. P. Bate inquired what advantage this was considered to pos- 
sess over Rowland’s reversible compressor, described in the last edition 
of Carpenter’s £ Microscope,’ pp. 295 and 6, which could be turned over 
without removing it from the Microscope ? 
Mr. Macer said that Rowland’s was placed so far from the condenser 
that it could not be used for work which required a high-power con- 
denser. 
The President thought the fact of Mr. Macer’s form being able to be 
used with a condenser was a very substantial advantage ; it certainly 
seemed to be a very useful piece of apparatus. Mr. Rousselet’s pattern 
was also a very practically useful instrument, and the thanks of the 
Society were due to both these gentlemen for bringing their devices 
under their notice. 
Surgeon Lieut. -Colonel Bate showed a very little known method of 
illuminating diatoms, which he called “ white ground.” The method is 
simple and only requires ordinary apparatus. The Microscope is 
arranged as for dark-ground illumination with the largest stop in the 
Abbe condenser on the substage, a 12 mm. apochromatic with compen- 
sating eye-piece 27 on the tube, and a Triceratium or other suitable 
diatom on the stage. 
Adjust as for best dark ground, and on moving out the arm carrying 
the stop (which should work rather stiffly), a point will be found where the 
dark ground just disappears and is replaced by creamy white ; the diatom 
is now seen strongly illuminated and glowing like fresh minted silver 
on a dull ivory ground, and as the shadows are rather strong the effect 
is exactly that of opaque illumination with a Lieberkiihn. There is no 
distortion, details are shown with remarkable clearness, and in stereo- 
scopic relief. The use of monochromatic light by means of a bottle of 
copper solution improves the effect. 
The President said they were all aware how clearly and strongly an 
object appeared to stand out under this class of illumination, the only 
question being whether the method did not produce some amount of 
distortion, and whether therefore the beauty of the effect was not 
obtained at the cost of some accuracy. 
Prof. F. Jeffrey Bell read a letter received from Captain Montgomery, 
of Ismont, Natal, describing the abundance of ticks in that colony. 
The President said there could be no doubt that the tick question 
