Optical and Actual Thickness of Microscopic Covering Glass. 269 
and by the method already described a fine miniature of the lines is 
thrown upon the field of view. 
Better still, I fix a parallel wire micrometer below, and throw 
the image of the wires upon the stage. The half inch, if the micro- 
meter wires be 5 inches below the stage, will diminish it nearly 
11 times ; if £ be used the scale will be diminished nearly twenty 
times, and the wires will appear to be separated about the 20th 
part of the actual space, which, however, can be previously ascer- 
tained. The great superiority of this method consists in the 
absolute steadiness and the preservation of the light and defini- 
tion. I have often found it quite hopeless to measure accurately 
minute bodies of difficult definition, whilst using the Jacksonian 
micrometer in the eye-piece, the intervening micrometer glass 
plates on which the divisions are drawn entirely obscuring and 
destroying a fine definition displayed at last, perhaps by very 
great pains. Further, the positive eye-piece micrometer, by the 
vibration it causes upon the least touch of the divided milled head, 
renders the distinct measurement a work of great difficulty. But 
by this method the micrometer lines can be reduced to any degree 
of fineness, and solid wires of any required diameter be substituted 
for the spider lines. 
The advantages are these. In observing a delicate object we are 
frequently changing eye-pieces or objectives, altering the cover 
adjustment, or using water or dry lens ; under these circumstances 
the final measurement of a delicate object, with great difficulty seen 
at all, requires to be done without any change whatever of the 
observing instrumentation. It is at that point that a delicate 
measurement is most desirable, when often the means fail us. But 
if we can apply a miniature micrometer imaged at once upon the field 
of view, the object is instantly obtained without disturbing the 
oculars or objectives. 
Remark. — A covered moderator lamp, a white cloud, or dif- 
fused illumination, is necessary to define the miniature micrometer 
properly, and a half-inch or inch object-glass for condenser. 
VIII. — On a Method of estimating the Optical and Actual Thickness 
of Microscopic Covering Glass. By G. W. Boyston-Pigott, 
M.A., &c. 
The importance of covering glass in microscopic research is daily 
becoming more evident : and having occasion to use a new 1-5 0th 
immersion of Powell and Lealand, the only one in existence, it 
became necessary to devise a means of readily measuring the 
x 2 
