BEALE, ON OBTAINING LITHOGRAPHS. 115 
consequence of the magnifying power of the object-glass not having been 
accurately ascertained, and an object said to be magnified in the same degree 
by two authorities is not unfrequeutly represented much larger by one than 
by the other. This arises from the magnifying power of the glasses not 
having been accurately ascertained. 
“1 cannot too strongly recommend all microscopic observers to ascertain 
for themselves the magnifying power of every object-glass, aud to prepare, in 
the manuer presently to be described, @ scale of measurement by which the 
dimensions of every object can be at once ascertained. 
“The inconvenience of not being acquainted with the number of diameters 
which any object represented in a drawing is magnified, has been often felt ; 
for without this it is impossible to judge of its real size. And, on the other 
hand, the annoyance of reading a long description of minute objects, differ- 
ing slightly in size from one another, the dimensions of which have been 
accurately noted, is very great; while no corresponding advantage is de- 
rived from such minute measurements. The text becomes occupied with a 
multitude of figures of but little interest to the reader. At the same time, 
it is very desirable that the careful observations of different persons should 
be readily comparable with each other. Elaborate researches are not un- 
frequently deprived of much of their value in consequence of measurements 
having been carelessly taken, or the magnifying power of the glasses 
wrongly expressed. 
“The plan of appending to every microscopical drawing a scale magnified 
in the same degree as the object represented, supersedes the necessity of 
giving measurements in the text, while it is free from any of the objections 
above referred to. I propose to describe briefly a very exact, and at the 
same time a very simple, method of applying scales to microscopical drawings. 
All the drawings illustrating the editor’s papers may be measured by the 
scales at the bottom of the page, and he strongly recommends all contri- 
butors to follow the same plan. 
“To carry out this it is necessary to ascertain the magnifying power of 
every object-glass, and to be provided with a stage micrometer divided into 
LO0ths and 1000ths of an inch. 
“ Mode of ascertaining the magnifying power of the object-glass.*—A glass 
micrometer divided into 100ths of -an mch is placed in the focus of the ob- 
ject-glass of the microscope, which is arranged horizontally. The neutral 
tint glass-reflector is fitted to the extremity of the eye-piece, and the light 
carefully arranged so as to render the micrometer lines distinctly visible. 
Care must, however, be taken that the distance from the object-glass to the 
reflector is the same as from the latter to the paper beneath it, upon which 
the magnified micrometer lines may now be traced. A four- or six-inch 
scale accurately divided into 10ths of an inch is now applied to the magnified 
LOOths of an inch, and the magnifying power of the glass is at once ascer- 
tained. Suppose each magnified 100th of an inch covers 1 inch, the mag- 
nifying power will be 100 diameters, if an inch and 3 tenths 130 diameters, 
if 4 tenths of an inch 40 diameters, and so on, each 10th of an inch corre- 
sponding to a magnifying power of ten times. 
“Tf we wish to ascertain the magnifying power of one of the higher object- 
glasses, a micrometer divided into 1000ths of an inch should be employed 
instead of the one just alluded to. In»this last case, each tenth of an inch 
* «This mode of measuring is alluded to in several works on the micro- 
scope, but the editor considers it sufficiently important to repeat here, 
especially as the drawings illustrating papers published in the ‘ Archives’ 
have been copied in this manner.” 
VO Vil. K 
