218 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
to 1*20 N.A.) is used in bright-field work without oil-immersion 
contact between the under-surface of the slide and the top of the bright- 
held condenser. As the light of an aperture greater than 1*00 N.A. 
cannot emerge from the condenser into air, it follows that not nearly 
all of the available aperture is employed. Of course when the resolu- 
tion of fine details is involved the higher aperture is of great importance,, 
but in order to be fully utilized the microscope slide must be in 
immersion contact with the top of the condenser. 
A New Form of Interferometer. — H. P. Waran ( Proc . Roy. Soc. y 
Series A, 100, 1922, A705 , 419). Instead of a thin plate of glass with 
its two surfaces worked optically true and parallel to one another, as in 
the Lummer and Gehrke interferometer, the author has devised an 
apparatus in which a thin layer of one liquid (water) boats on a layer 
of another liquid (mercury), thus adopting an idea which was used by 
the late Lord Rayleigh. Great trouble was experienced from earth- 
tremors, and extraordinary precautions were necessary to eliminate 
them, but in the end all difficulties were surmounted. The source of 
light used was that of a glass mercury-vapour lamp filtered through a 
green filter to get the intense monochromatic radiations of wave-length 
5460*7 A.U. Analysis of the light transmitted by this filter showed 
that the two yellow lines were still present faintly, while the other lines 
were practically eliminated. Lamps of convenient form were success- 
fully constructed by the author. The trough, which contains the 
interferometer liquids, is a closed rectangular vessel, 40 x 10 x 6 cm. in 
size. One end is formed by a prism of equilateral section, attached to 
the sides of the trough, with one of its surfaces inclined at an angle of 
13° (for water) to the vertical. The collimated beam, which is reflected 
upwards, enters the hrst face of the prism normally, and after total 
reflection at the second face enters the liquid after passing out normal 
to the third face. The beam of light is successively reflected at the 
upper and lower surfaces of the water layer, and on emergence through 
a thin mica window is caught in a telescope and examined for the 
formation of the system of interference bands. It is advantageous to 
have the individual bands widely separated to secure high resolving 
powers, and hence the thinner the water layer the better. The fringes 
were found to be remarkably sharp on quiet days. A. N. I). 
The Massive Model Microscope.— This instrument, which was 
designed for the National Institute of Medical Research, is of a more 
massive and solid design than has hitherto been attempted in microscope 
construction. This, combined with extreme delicacy in the adjustments, 
enables it to be used with high powers with almost complete freedom 
from flexure and the effects of vibration. The limb is a massive brass 
casting with a machined fitting for the slow motion at one end and a 
dovetailed fitting for the substage apparatus at the other end. The 
stage is a heavy casting with side brackets below fixed rigidly and 
permanently to the limb, so that the whole forms one solid element. 
The pillar and base are very heavy and rigid, and a short post on the 
base forms a rest for the limb when it is placed with the body in a 
horizontal position for photo- micrography. The substage has focusing 
