ON PHOTO-MICROGRAPHY. 201 



purposes are penetration and flatness of field. 

 These qualities are of greater importance in lenses 

 for photographic than for visual purposes, for in 

 viewing an object under the microscope the observer 

 has the power of focussing in rapid succession, and 

 by imperceptible gradations, points at different 

 depths and different distances from the centre of 

 the field; but a photograph represents only such 

 structures as were in focus at the time of exposure, 

 and once taken, the focus is unalterable. It is 

 therefore desirable to secure as great a depth of 

 focus and as flat a field as possible qualities which 

 are incompatible with large apertures. 



This brief chapter is merely elementary and 

 intended as a guide to beginners. Those who desire 

 to thoroughly acquire and successfully practise this 

 fascinating art, and to learn the details of the most 

 recent processes and manipulations, as well as the 

 use and value of apochromatic objectives and of 

 all other modern appliances and apparatus, should 

 obtain " PRACTICAL PHOTO-MICROGRAPHY" recently 

 issued by that master of the art, Mr. Andrew 

 Pringle, and published by Messrs. Iliffe and Son, 

 St. Bride Street, London, E.G. 



THE END. 



14 



