470 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and the edge of the liquid blueish-green, while the reverse is the case 
if adjustment be made on the lower plane C D. 
These colour appearances are best obtained with a system of con- 
siderable focal length, e.g. with the Zeiss system a a and the weak apo- 
chromatic system of 16 mm. focal length. 
The phenomenon is of practical importance for the determination of 
the refractive indices of Microscopic objects on which no plane faces can 
be cut. Out of a number of mixtures of liquids with refractive indices 
differing by about five units in the third decimal place, one is sought 
which shows the coloured edges most distinctly when the object is 
immersed in it. In this case the values of w D for the solid and the 
liquid are not very different. Observation is then made in sodium light, 
and liquids of various refractive indices are tried until one is obtained 
in which the boundary completely disappears. 
(6) Miscellaneous. 
The Planktonikrit, a Centrifugal Apparatus for the Volumetric 
Estimation of the Food-Supply of Oysters and other Aquatic 
Animals.* — Dr. C. S. Dolley referring to the work of Hensen, Haeckel, 
and others on planktonology, explains the importance of a quantitative 
determination of the primitive food-supply of marine animals. Since 
oysters, clams, and mussels depend practically upon diatomaceous food, 
determinations of the bulk of diatoms for each cubic metre of regions 
abounding in Molluscan fauna should be made, and used as standards for 
estimating the value of neighbouring waters. The ostreiculturist should 
also have a quick and accurate method for determining the amount of 
plankton in the water of his parks. For this purpose the author uses a 
centrifugal apparatus, instead of the old method of counting the indi- 
viduals. The apparatus consists of a series of geared wheels, by which 
an upright shaft is caused to revolve up to a speed of 8000 revolutions 
a minute. To the upright is attached a frame carrying two funnel- 
shaped receptacles of 1 litre capacity each. The main portion of each 
of these consists of tinned copper. To this is attached the stem of the i 
funnel, which is formed of a heavy glass tube of 15 cm. outside diameter 
with a central bore of 2J to 5 mm. These glasses are protected by a 
cover. 
The Microscope as a Guide in Medicine. f — Dr. C. G. Kuhlman 
has a remarkable article on this subject, the summary of which we would 
not dare to give otherwise than in his own words. This author finds 
that he is justified in arriving at the following conclusions: — 
(1) That the Microscope is of little or no value as a guide in the 
study of medicine at the present date, because every known pathologic 
condition and change can be recognised microscopically. Pathologic 
conditions and changes cannot be differentiated microscopically. Owing 
to defects in Microscopes, defects in sights of students, unsuccessful 
preparation and manipulation of objects and Microscopes, modern illus- 
trated text-books are far superior as object lessons. 
* Proc. Acad. Nat. Sci. Philadelphia, 1896, pp. 276 80. 
f St. Louis Medical and Surgical Journal, lxx. (1896) pp. 201-9. 
