'252 SUMMARY OF CURKENT RESEARCHES RELATING TO 



microscopic method suitable for use on board ship, in attaining this 

 degree of accuracy. He passes a beam of monochromatic Hght through 

 the sHt of a colHmator on a rectangular trough with parallel faces, 

 separated into two parts by a diagonal glass partition. The beam 

 impinges at right angles on one of the faces of the trough, which thus 

 forms a system of two opposite, equi-angled prisms. If these prisms 

 contain liquids of the same index the ray traverses their system without 

 undergoing deviation. But deviation ensues if the liquid in one differs 

 from that in the other. If A be the common angle of the two prisms 

 the deviation caused by the first liquid (index n) i&d = {n — 1) A ; that, 

 in the opposite sense, by the second liquid (index n') is d' = (n' — l) A. 

 The resultant deviation is their difference, or A = (n — n') A. For 

 measuring this deviation the luminous beam, on emerging from the 

 trough, is received on an achromatic lens, which gives a real image of 

 the collimator slit. This image is then examined by a Microscope 

 provided with an ocular fitted with a micrometric screw. One of the 

 liquids is a standard one of known density and consequent salinity, the 

 other is the sample. 



Benzoline for Microscope Lamps.* — S. C. A. has tried benzoline in 

 place of paraffin with satisfactory results. The metal fount should be 

 stuffed with cotton-wool — not too tightly — then fill with benzoline and 

 allow to stand until the wool is thoroughly saturated. Any superfluous 

 benzoline should be poured off. See that the wick is in contact with 

 the cotton-wool, and that it does not become twisted when screwing on 

 the burner. A lamp charged in this manner can be carried in any 

 position ; there is no smell, and the light is white. The lamp should not 

 be filled in proximity to a naked light. 



(4) Photomicrography. 



Photography of Eye Specimens.! — R. H. Elliot gives the following 

 points and says that if a good photograph of a sectioned eye is to be 

 obtained attention to certain points is essential : — 1 . The eye must lie 

 completely under water. 2. The source of light must be good and ^ven. 

 3. The camera must be placed vertically above the object in order to 

 avoid reflexes. 4. The object photographed must be so placed that its 

 image will occupy the centre of the plate, and adjustment should be 

 available to secure this end with a minimum of inconvenience. 5. A 

 simple arrangement is necessary to fix the eye section in position during 

 the whole period of exposure. 6. To save unnecessary retouching the 

 object should be photographed upon a dark and even surface, so that 

 the background should in no way disturb the attention of anyone 

 looking at the picture. 7. A suitable camera should be selected. 

 8. The question of exposure must be very carefully studied. The author, 

 after discussing these points in detail, appends an addendum on magic- 

 lantern slide-making. 



* English Mechanic, March 2, 1917, p. 105. 



X Proc. Eoy. Soc. Med., x. (1917) pp. 7-13 (Opthal. sect.). 



