410 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
nre generally diatoms, and the magnification 1000 diameters. The 
Microscope is placed in its horizontal position, and the milled head of 
its fine-adjustment brought into gear with the focusing-rod by means 
of a piece of thin whipcord. The heliostat is placed in front, on a 
wooden stand, which carries also the fixed mirror and the alum-cell for 
absorbing heat-rays. Care has to be taken that the optical axis of the 
whole apparatus is directed due south, which is insured by the end of 
the board upon which it stands being cut at such an angle that when 
this end is placed against the plate glass of the window, all is in right 
direction. 
The first operation is to accurately centre the achromatic condenser, 
using a two-thirds objective and regulating the diaphragm so that its 
opening may be a little smaller than the field ; next, to centre the further 
diaphragm at the end of the brass plate ; and afterwards, removing the 
movable mirror from the heliostat, to ascertain that the spindle appears 
precisely end on and in the centre of the field, which it should do if the 
heliostat has been properly placed. Exactness in this last adjustment 
is necessary, otherwise the beam of sunlight will not be motionless. 
The movable mirror is then replaced on the spindle, and set to reflect 
the image of the sun in the centre of the field. At this stage the eye 
must be protected by a dark-coloured glass being placed below the con- 
denser. The object being placed on the stage, brought into the centre 
of the field and focused, the condenser has next to be focused to throw 
the sun’s image exactly in the plane of the object. Sharpness of the 
ultimate image upon the ground glass cannot be secured without this. 
Changing the objective to a one-sixth (4 mm.) I next measure the 
thickness of the cover-glass, or rather the distance between that plane 
of the object which it is desired to photograph and the upper surface 
of the cover-glass, by means of the fine-adjustment screw. The purpose of 
this is twofold. First, to facilitate cover-correction ; secondly, to ascer- 
tain whether the 2 mm. object-glass, which is now put on, can get down 
to it, for its front lens is rather more than a hemisphere, and the mount 
in which it is set is so extremely thin that it has hardly any grip on the 
lens, and the slightest pressure suffices to displace it. My glass, with a 
distance of 0*18 mm. between the object-plane and the upper surface of 
the cover-glass, requires no correction. For a thinner cover — and the 
covers of English-mounted slides generally are thinner — correction is 
effected by lengthening the tube-length ; for a thicker cover, by shortening 
it. Lastly, the illuminating cone thrown by the condenser has to be 
regulated. You are probably aware that there is great controversy as 
to what this should be in order to produce a “ true ” image. My expe- 
rience is that the width of the cone should vary according to the nature 
of the object and the quality of the object-glass. Too narrow a cone 
produces diffraction fringes, that bane of photomicrography ; too wide 
a cone, even, I think, with the best objectives hitherto made, produces 
haze. With thin “test objects” I find my own glass works best when 
about two-thirds of its back lens is filled with light ; for thick objects, 
I get the best results with a somewhat narrower cone. Of one thing I 
am convinced, that to get true images, the cone, whether it be wide or 
narrow, must be absolutely axial. Even a very slight obliquity renders 
the images unreliable. 
