12 PHOTO- MICROGRAPHY 



earth, for it is never still for more (apparently of course) than a few seconds together. 

 That is to say, when employing it just as the light is made central in a manner to be 

 hereafter explained, the sun apparently moves and does not retain its position in 

 the field of view long enough to enable a photograph to be taken with its use. The 

 special form of a^jparatus to keep it so is called a heliostat and is shown in Figs. 7 

 and 8. We believe the form (Fig. 7) which is sold by Baker, of Holborn, answers 

 the purpose very well indeed. Other forms are made, notably one after Prazmowski, 

 by Messrs. Besu, Hauser & Co. of Paris, and by Hartnack in Potsdam. Messrs. 

 Watson & Son of Holborn also supply an ingenious arrangement devised by Dr. 

 Johnson Stoney, shown in Fig. 8 ; neither of these two are personally known to us, but 

 by repute they are generally acknowledged to be exceedingly good although rather 

 costly. Any kind of heliostat must be made for the latitude of the place, and as full 

 directions for their setting up and use are issued by each maker a detailed description 

 need not be here given. 



