as an Aid to High-power Definition. 
189 
made of two pieces of thin plate glass about 2J inches square, held 
apart by thin strips of plate glass, or by a square of plate glass 
suitably drilled. The point is to obtain a layer of the blue solution 
about §• of an inch thick between two parallel planes of plate glass. 
The best cement for the purpose according to my experience is old 
Canada balsam applied hot; but many other devices may be 
employed. The solution is made by saturating strong aqua 
ammonia with sulphate of copper, and should be strained or 
filtered so as to be free from all solid particles. A sheet of fine 
blue glass may be substituted for the ammonio-sulphate cell, but 
only with tolerable results ; at least, I have never had a sample of 
blue glass which was of just the right colour. 
The selection of the best condenser for high-power definition is 
a matter which has recently elicited much discussion. In a general 
way I may say that any condenser will do its best work under the 
conditions above indicated if skilfully used. For the benefit of 
those who possess first-class stands but have never purchased an 
achromatic condenser, it may be stated that almost any objective 
suitably mounted on the secondary stage can be made to answer 
instead, the best results being attained when the angle of aperture 
of the objective thus used is rather less than that of the one 
employed to magnify the preparation, and when the secondary 
stage is capable of being centred or decentred at pleasure by screws 
working at right angles to each other. An ordinary low-power 
objective (of 1 to 8 inches focal length), mounted on a separate 
stand and used to throw the light obliquely as already described, is, 
however, perhaps the most convenient and efficient mode of illu- 
minating line test-objects with high powers. 
As to the objectives suitable for monochromatic illumination, the 
best compound objectives of some first-class maker should be selected. 
It is a mistake to suppose, as some have done, that a single lens can 
be substituted for the modern carefully-corrected compound objec- 
tive, even if the pure monochromatic light of a narrow portion of 
the solar spectrum as obtained by a prism were employed. For the 
objective always requires to be corrected for spherical aberration, 
and in the case of high powers must be provided with a screw- 
collar to modify the distance between the posterior combination and 
the front one in accordance with the different thickness of the 
covering glass of the preparation. Now practically the spherical 
aberration is best corrected by the just combination of crown and 
flint glass, and combinations very nearly the same as those employed 
for white light would still be necessary if the objective were made 
for exclusive use with monochromatic illumination. 
Under these circumstances I do not recommend the use of 
monochromatic illumination for low or medium powers except 
when photographs are to be made. It is only as an aid to high- 
