TESTING MICROSCOPE OBJECTIVES. 149 
Flatness of field and penetration are two qualities of which I can 
say but little. Many of the best objectives have a very flat field, 
but I have never yet seen one in which the definition was really 
good to the extreme edge. It is easily determined by examining 
any flat, well-marked object, such as a section of echinus spine with 
the lower powers, and a slide of evenly spread blood-corpuscles 
with the higher ones. 
Working distance is of especial value in high powers. It is the 
distance of the object from the front lens when in focus. It can 
only be ascertained by actual measurement. For this purpose a 
scale and vernier on the ways and the body are very desirable, but 
I have never yet seen a micrometer screw sufficiently good to cor- 
rectly measure the working distance of the higher powers. ‘The best 
way of measuring the greatest working distance is by having a num- 
ber of cover glasses of various thicknesses, accurately measured, and 
see which is the thickest that the objective can be made to focus 
through, using the ten-inch body. It will be found that the actual 
distance between the object and the objective will increase with an 
increase in thickness of cover-glass or the density of immersion 
fluid. 
The magnifying power of the objective is a matter well worth 
knowing. Many objectives are underrated ; for example, a recent 
qo Which I measured only gave 84 diameters at its greatest power, 
while another 4, gave the unusual power of 120 diameters, thereby 
being a true 5. 
The correct point from which to measure the power, is the optical 
centre, but as this changes with every change of immersion fluid or 
length of body, it is not practicable, hence the front surface of the 
front lens is generally taken as the standard, and the image is 
measured at a distance of ro inches from this point. ‘The best way 
of doing this is by placing a micrometer on the stage and a piece of 
finely ground glass at the end of the body instead of the eye-piece. 
The distance of the ground glass from the front of the objective 
should be ten inches. If the micrometer be now illuminated by 
very intense lamp light, or in the case of the higher powers by sun- 
light, and carefully focussed, the image of the lines will be seen on 
the ground glass. Their distance apart may then be dotted on the 
ground glass and then carefully measured. If the distance apart 
of the dots upon the ground glass be now divided by the real dis- 
tance of the lines on the micrometer, the linear amplification will 
be the result. 
The power of the eye-piece may now be determined from that of 
the objective. The eye-piece should be placed in the tube, with 
its field-lens in the case of a negative eye-piece and its focal point 
in the case of a positive eye-piece, at a distance of ten inches from 
the front lens of the objective. The power of the microscope 
