EVENINGS WITH THE MICROSCOPE. 133 
2. Abbe’s test plate. 
The student’s microscope perhaps may be a binocular, but most 
probably it will be one of those working instruments illustrated on 
page 38 of “Practical Microscopy.” We will suppose it to be 
fitted with an inch objective, and also with a 3th, both of moderate 
angles of aperture. 
The first exercise must be to measure the objectives and oculars, 
z.é., their magnifying power, and the state of their corrections ; the 
working distance can only be correctly measured by special appli- 
ances which need not be mentioned here yet. 
The stage micrometer, when viewed with the combined objective 
and ocular, will show us the flatness of field or otherwise, and this 
should be studied on all lengths of tube from seven to eleven 
inches, so that by the appearance of the object, the observer may 
know whether, in order to produce the best effect, the tube of the 
instrument should be shortened or lengthened from the normal. 
Some objectives are very sensitive to alterations in the length of 
tube, and will only work well at one set length. 
Let us now proceed to measure the magnifying power of our 
one-inch objective. For this purpose we must place the micrometer 
on the stage, and by preference Ramsden’s eye-piece micrometer 
in its place at the upper extremity of the tube. The micrometer 
in this eye-piece is always below the field lens, and is generally 5% 
of an inch in length, each 4 being graduated into ten parts. The 
length of tube must now be adjusted and the stage micrometer 
brought to a focus, so that the distance between the two micro- 
meters (that of the stage and of the ocular) is rather more than ten 
inches, the distance may be variable, but must permit of accurate 
measurement, say for most practical purposes to the one-tenth of 
an inch. If z represents the magnifying power at the distance J, 
the following formula will give the true denomination of the lens: 
etd 
(7+ 1)? 
Real focal length = 
To give an example: Suppose the one-hundredth space of the 
stage micrometer be magnified so that it occupies the space of 
twelve of the hundredth lines of the ocular micrometer, it will be 
clear that the amplification is twelve diameters, the distance 
between the two micrometers is ten inches. Multiplying these 
together, we get 12 x 10=120, place this as the numerator of the 
fraction ; the number of diameters magnified must now be added 
to by one, and the whole squared to form the fractional denomi- 
nator, in this case (12 +1)” =169+1430= "71, or roughly ,% of an 
inch. 
As a guide to the student, we give the measurements of several 
one-inch objectives which we have examined :— 
