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VII. — On a Simple Form of Ileliostat, and its Application to 
Photomicrography. 
By Thomas Comber, F.L.S, 
{Read 21s£ May , 1890.) 
Your Secretary has asked me to give your Society a detailed de- 
scription of the apparatus I use for photomicrography, and of my 
method of working ; but it appears to me that it will be simpler and 
shorter, and at the same time answer every purpose, if I merely explain 
those features in which my mode of working differs from that which 
I believe is generally adopted by others, and is probably sufficiently well 
known. The general nature of the arrangement will he apparent 
from the woodcuts. 
The two main objects that I have endeavoured to attain have 
been, firstly, a means of sunlight illumination, easily applied, quickly 
adjusted, and simple in construction so as not to be liable to get out 
of order ; and secondly, an arrangement which admits of convenient 
and comfortable eye-observation, for the purpose of arranging the 
object and adjusting cover-correction, before the camera is attached to 
the Microscope. 
So far as my experience goes, for high magnification — other 
things being equal, both as regards objectives and manipulative skill — 
better results can be obtained by sunlight than by any other kind of 
illumination. The photomicrographs produced by Mr. Nelson and 
other of your members by oxyhydrogen light may be superior to 
what others have produced by sunlight ; but this is due to their 
superior optical appliances and greater skill as microscopists, which 
more than compensates for what I cannot help regarding as inferior 
illumination. The same operator, using the same lenses, will, I am 
confident, produce better results by sunlight than by any artificial 
illumination. 
The reasons sunlight has been so little used in this country are 
probably (1) the uncertainty of our climate; (2) the fact that many 
of our microscopists work chiefly in the evening ; and (3) the com- 
plicated nature of the heliostats obtainable, which renders them very 
liable to get out of order, and so difficult to adjust that, when sunlight 
is available, much time is lost in setting up the apparatus ; and, con- 
sequently, before everything is in working order, the sun may too 
often become clouded. The last objection is aggravated by the heliostat 
being usually placed a considerable distance from the Microscope, and 
sometimes even outside a window ; and, as any error in the action of 
the heliostat is increased in proportion to the distance, it has been 
found almost impossible to keep the illuminating beam unchanged by 
the motion of the sun. 
To avoid this difficulty, I place the heliostat inside the window, 
