ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
663 
In my own experience I have found a broad cone of illumination 
unsatisfactory, for the same reason that I have found oblique light in 
one direction unsatisfactory. It is almost impossible to centre the sub- 
stage condenser so accurately that a wide cone can be trusted to be 
central. If you centre it by examination with a low power, it is almost 
certain that it will not be centered for a high power, for two objectives 
are rarely centered alike. The field, under a magnification of 1750 
which Mr. Smith has commonly used, is so small that the least decentering 
will illuminate it only by the oblique rays from one side of the cone, 
and we then immediately get diffraction effects, I am bound in candour 
to say that in most of Mr. Smith’s prints I recognize similar effects to 
those which, in my own work, I attribute to oblique light. It may be 
that, with improved contrivances to secure exact centering of objective 
and condenser, we shall find advantages in the use of the wide cone. I 
speak now only of my own experience under existing methods. The 
slightest turn of the mirror on its axis will change light from central to 
oblique ; and I suppose we are all in the habit of doing this, so as pur- 
posely to throw light through one side or segment of the condenser for 
the purpose of studying the effect on an object of the changing direction 
of illumination. So unstable a source of light prevents our knowing 
very exactly when the light is strictly central, and makes it hard to re- 
turn to any exact condition from which we have departed even a little. 
These considerations have kept me (perhaps mistakenly) in the practice 
of using the narrow cone of light for photography, reserving my oblique 
light for special resolutions of striation and for the professed study of 
changing effects. 
Similar reasons have made me distrustful of dry mounts when high 
powers are to be used upon any but the thinnest objects. Refraction, 
and attendant diffraction, are so increased with increase of index, or 
rather increased difference of index, that it has grown to be a maxim 
with me to have the mounting medium and the object as near alike in 
index as is consistent with the discrimination of structure. The pale 
images of transparent objects are those I find most truthful, for paleness 
is consistent with good definition and resolution, whilst the brilliant 
pictures are apt to be glittering deceptions. I fully admit, however, 
that it may well be that with improved glasses we may add to the extent 
of details visible upon a surface, like that of a diatom- shell, and that it 
is possible that mounting in most media would obliterate the finest of 
these details. To a certain extent we are all familiar with this. A 
rather coarse dry shell like P. balticum will have its details instantly 
obliterated if water from the immersion of our objective penetrates 
beneath the cover-glass. Mr. Smith’s print No. 50 might pass as an 
excellent reproduction of this effect, the fluid passing along the struc- 
tural lines, obliterating part and leaving part. 
But when full weight has been given to all these things, and we have 
put aside those of Mr. Smith’s long and beautiful series of photographs 
fragment of P. angulatum , showing partial removal of one film, and fracture through 
dots over a large space.” In preparing this paper I have repeated the examination 
with the objective named, and find the distance between upper and lower film easily 
appreciable in focusing. 
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