ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
263 
with the refractive index of the resinated cedar oil, but without the ob- 
noxious quality of the fluid that acts on the objective mounting? For 
these learned men the problem should be an easy one. The maker of 
the dangerous glycerin mixture can surely make something as good. I 
hope he will never make anything quite so bad, although in its optical 
action it is as nearly perfect as need be wished. Its hunger for metal 
is the fatal objection to it. 
Upon the optical action of the immersion fluid depends the optical 
action of the homogeneous-immersion objective. If the former is not of 
the proper index, the microscopist may deceive himself by believing that 
his objectives are giving him the best possible results ; or if they seem 
to be optically defective he should remember that the fault may be in 
the fluid supplied by the dealer. The optician should place at the 
disposal of every microscopist some simple device by which the refractive 
index of the immersion medium may be ascertained. Zeiss sends out 
for this purpose a wedge of glass, which, when used as directed, gives 
the desired information. Prof. H. L. Smith’s device is not obtainable, 
and that of the German optician can be had, I suppose, only by buying 
one of his homogeneous-immersion objectives. Without some such 
means, the microscopist can never know whether he is getting the best 
work from the objective or not, unless he attempt to resolve the proper 
diatom every time he begins to use a fresh supply of immersion 
medium, a method that would be time-consuming, and should be unne- 
cessary. While with the improper fluid he may get moderately good 
results, with a medium with the correct refractive index he will get the 
best that the objective can give, provided of course the lens be properly 
manipulated.” 
(3) Illuminating: and other Apparatus. 
A new Modification of the Abbe Drawing Apparatus.* — Dr. W. 
Bernhard deprecates the discredit into which microscopic drawing has 
fallen, owing to the enormous advances recently made in photomicro- 
graphy. Without denying the immense practical advantages of photo- 
graphy, he considers that there are many cases in which the objectivity of 
the photograph is not desirable. In microscopic investigations we often 
require to know what the observer really saw, not what he could have 
seen, and it is only a drawing which can give expression to such subjec- 
tive observations. 
The cause of the complaints brought against drawing apparatus on 
the ground of indistinctness of the image and of the point of the pencil 
is not due to any defects in the optical parts of the apparatus, but mainly 
to a want of proper regulation of the light. All drawing apparatus have 
this in common, that the plane of the image is projected upon the plane 
of the drawing. With unequal intensities of the light of the two surfaces, 
it is clear that the more intense will have the effect of making the less 
indistinct. In order to see clearly at the same time the plane of the 
drawing and point of the pencil on the one hand, and the Microscope 
image on the other, the intensity of the light must be made the same for 
each. This is effected by reducing the intensity of the light either of 
the imago or of the plane of the drawing. 
* ZeitscLir. f. Wiss. Mikr., viii. (1891) pp. 291-5. 
