200 MEMORANDA. 
for light to pass over the object and be reflected back again 
to the point of adhesion with the glass. If the aquarium 
has glass sides the same rule will apply, which the diagram 
will illustrate, by considering the tank, condensing lens, &c., 
to be in plan and placed at the side instead of the top. 
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e. is not a lens as shown, but a short thick line representing the place 
of the object. 
Of course, at a proper incidence, total internal reflection will 
also take place from the top surface of the water alone, but 
I do not see how this can be made available. 
Object-glasses of a focus as short as half an inch may be 
used in the aquarium by the following method: <A tube is 
to be made to slide over the front of the object-glass, and 
extending back for two or three inches over the body of the 
microscope. A very small glass prism, either right angled or 
equilateral, is cemented or otherwise fixed water-tight in 
the end of the tube, with one of its faces nearly in contact 
with the front lens. The microscope may be immersed in 
the water, and used either vertically or inclined, as deep as 
the “ water-boot” will allow. The diagram will explain the 
arrangement. If the work is neatly put together, consider- 
able focal distance may be obtained in front of the prism, 
as for a half-inch object-glass it need not be more than 
one eighth of an inch square, on its faces. The water 
must not be allowed to get to the back of the prism, as 
it will prevent total reflection therefrom, except at very 
oblique incidences. For a direct forward view under water, 
the prism may be replaced by a small disc of thin glass at 
right angles to the ae axis ; this is an old suggestion br 
Dr. Goring. —F. H. Wennam. 
