FOCUSING THE IMAGE IN PHOTOMICROGRAPHY. 233 
FOCUSING THE 
IMAGE IN PHOTOMICROGRAPHY. 
i pursuing the pleasing art of photomicrography there is no 
doubt that every operator has at one time or another of his ex- 
perience had great difficulty in satisfying himself of the necessary 
sharpness of the image on the ground glass. Veterans of the art 
are known to have constructed appliances by means of which 
many of the difficulties may be bridged over ; but the tyro is, as a 
rule, unacquainted with these so-called “little dodges,” and there- 
fore we purpose devoting a little of our space to the description of 
several methods for getting the exact focus of microscopic objects 
on the ground glass. 
Dealing with low powers is not so troublesome as with high ones, 
as there is always sufficient light to enable a tolerably good focus 
to be obtained ; but with high powers, and consequent loss of light, 
it requires all the skill at the operator’s command to obtain even a 
passable picture in focusing by means of the ground glass alone. It 
has been the practice with some to use the finest ground glass 
obtainable, and to oil this over with olive oil, while others have 
discarded the use of ground glass as a focusing medium, and have 
thrown the pictures upon fine Bristol card-board placed in ex- 
actly the same plane subsequently occupied by the sensitive surface 
of the plate. 
There is no doubt that the oiled ground glass enables the picture 
to be more accurately focused than when an unprepared surface is 
employed, but the want of light in the case of high powers is a dif 
ficulty not dealt with by this method. 
Some years ago, Mr. J. B. Dancer described to us his method, 
which is as follows :—Draw two lines over the roughened surface 
of the ground glass from corner to corner, with a writing diamond, 
and in the centre, where the lines cross, cement a thin cover glass, 
three quarters of an inch in diameter, with balsam and benzol. 
This produces a transparent circle, and as aids other circles of a 
similar character may be dotted over the plate in the portion 
usually occupied by the picture. 
Upon throwing the enlarged image upon a ground glass prepared 
as above, a little effort will enable the operator to distinguish the 
details of the picture upon the transparent portion, and in many 
cases, without any further aid, an exceedingly sharp focus may be 
obtained. In many cases, however, it is better to use an auxiliary 
microscope to examine this image on the transparent circle. Such 
an auxiliary microscope may be easily constructed : a piece of brass 
tube to hold the A ocular at its upper end, while the lower end is 
