108 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
quotient of the liquid used (as measured by the burette) by the area 
of the cylindrical vessel gives the thickness required. If an arrange- 
ment be attached to the burette by which the liquid can be drawn back 
into it, a great number of adjustments may be made for the same deter- 
mination. Mercury is the best liquid to use, and in this case a rod of 
the form C L is employed, but in the case of water, one of the form C. 
The exactness of the determination naturally depends on the correct 
adjustment of this rod ; it must stand vertically and must not rotate 
during a determination ; the glass plate also on which the point a rests 
must be exactly plane and horizontal. According to the author, when 
these conditions are fulfilled the new spherometer is in no way inferior 
in exactness to the ordinary instrument with micrometer screw. By a 
number of adjustments of the same level an exactness up to 0*001 mm. 
was effected. 
Chimney for a Microscope Lamp.— Mr. Nelson writes: — “This 
chimney, which Messrs. Watson have made from my designs, would at 
first sight appear to be an elaboration of the existing illuminating 
apparatus for a Microscope, whereas it is an attempt at simplification. 
While working with the superstage illuminator of the “ Farmer’s ” 
Microscope, it occurred to me that an equiconvex lens silvered mirror 
would form an excellent parallelizer for a Microscope lamp. 
A reflector to parallelize a wide-angled beam can be constructed on 
that principle with shallow curves, it is therefore apparent that the 
aberrations will be less than that of a single plano-convex lens,* whoso 
ratio of aperture to focus is the same. 
Fig. 6. 
The adaptation of such a mirror to the chimney itself was only one 
step further. The chimney, fig. 6, is the same as my ordinary one with 
a horizontal tube A A inserted, which is of course cut away inside the 
* Calculation shows that it is less than a half. 
