SOUECES OF ILLUMINATION 



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then available were not to be compared in sensitiveness to 

 those of to-day and the optical appliances were relatively crude. 

 The lenses were usually of small aperture, not so well polished, 

 and made of glass of less transparency than at present. 



A heliostat designed by Dr. G. Johnston Stoney, which is 

 particularly suited to the requirements of the physicist and 

 photo-micrographer, is made by Messrs. W. Watson & Sons of 

 High Holborn, and is throughout of first-rate construction. 



FIG. 35.- Simple Form of Heliostat. 



Fine adjustment for setting the instrument in its precise 

 position is afforded by means of rackwork and pinion and 

 tangent screw. 



A possible substitute for the heliostat where its cost is 

 prohibitive, or where for other reasons it is unobtainable, is 

 the arrangement suggested in the diagram, Fig. 35. Sunlight 

 is allowed to enter the room in which the work is being done, 

 in the direction as shown by the dotted lines S. The rays will, 

 of course, be parallel. They strike either on a mirror, which 

 should have a silvered surface, or preferably on a reflecting 

 prism A, and are thence reflected in the direction E, the latter 

 representing the sub-stage optical system of the microscope. 



