EVENINGS WITH THE MICROSCOPE. 135 
The student will now see how to go to work in order to arrive 
at a correct knowledge of the optical portion of the instrument, 
and if he varies these experiments as far as he is able, he will sub- 
sequently find he has not worked in vain. 
We must now turn our attention to the Abbe test plate, as made 
by Carl Zeiss of Jena. This plate is composed of a series of ruled 
bands on covers of varying thickness, which are as follows :—.o09, .12, 
15, .18, .21, .24 millimeter. These covers are coated with a thin 
film of silver, and groups of lines ruled upon them; the ruled sides 
are then cemented with balsam to the polished slips. Each band is 
composed of ten lines ruled at about ,4, of an inch apart, and when 
viewed with a good objective presents a series of sharply defined 
black and white stripes, opaque and clear lines alternating at 
close intervals and lying absolutely in the same plane, so that no 
deviation can occur in the course of pencils of light transmitted 
through it. 
A short and ready method of testing approximately any objective 
is recommended by Professor Abbe, as it is applicable to all 
instruments without requiring any apparatus except the test object 
already described. This may be briefly explained as follows :— 
First, focus the test plate with central illuminating rays, then 
withdraw the eye-piece, and turn aside the mirror so as to give the 
utmost obliquity of illumination, which the objective under trial 
will admit of. This will be best determined by looking down the 
tube of the microscope whilst moving. the mirror, and observing 
when the elliptic image of light reflected from it, reaches the 
peripheral edge of the field. As soon as this is done replace the 
eye-piece, and examine afresh the object plate zzthout altering the 
focus, If the objective be perfectly corrected the groups of lines 
will be seen with as sharply defined edge as before, and the colours 
of the edges must, as before, appear only as those of the secondary 
spectrum in narrow and pure outline. Defective correction is 
revealed when this sharp definition fails, and the lines appear 
misty and overspread with colour, or when az alteration of focus is 
necessary to get better definition, and colours confuse the images. 
A test image of this kind at once lays bare in all particulars the 
whole state of correction of the microscope, it being of course 
assumed that the observer knows how to observe, and what to 
look for. 
We have not space enough at our command this month to show 
how objectives can be critically tested by means of this plate and 
special diaphragms, so therefore let this exercise stand over for our 
second evening. 
( Zo be continued. ) 
