458 MONOCHROMATIC SUNLIGHT. 
ordinary Jens, or still better by a low power objective mounted 
like a bull’s-eye lens on a separate stand. 
For prolonged observation, however, the motion of the sun 
will render it necessary to readjust the mirror from time to time, _ 
and the use of a heliostat becomes desirable. This gives the 
most ‘satisfactory results no doubt, but the cost of the heliostat 
will, of course, prevent it from coming into general use except 
among those who desire to photograph what they observe, and the 
simpler methods above detailed will answer very satisfactorily for 
every other purpose. a 
The ammonio-sulphate cell used in either method should be x 
made of two pieces of thin plate glass about two and a half 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 4 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 brati 
r an 
` A sheet of fine blue glass may be substituted for 
sulphate cell but only with tolerable results; at least, pak 
never had a sample of blue glass which was of just the right color: 
The selection of the best condenser for high power aa ; 
a matter which has recently elicited much discussion. an ; 
general way I may say that any condenser will do its best ae 
under the conditions above indicated if skilfully used. For ee 
benefit of those who possess first class stands but have never ; ai 
chased an achromatic condenser it may be stated that almost a 
objective suitably mounted on the secondary stage can be m 
answer instead, the best results being attained wh + of the 
aperture of the objective thus used is rather less than pee 
one employed to magnify the preparation, and when the ' gems 
stage is capable of being centred or decentred at nee low 
screws working at right angles to each other. An ordinate 
_ power objective (of one to three inches focal length) pe x 
a separate stand and used to throw the light obligo t pe 
described is, however, perhaps the most convenient ane 
mode of illuminating lined test-objects with high ean 7 
As to the objectives suitable for monochromatic 1 
