ON PHOTO-MICROGRAPHY. 189 



of course, to intercept as large a quantity of light 

 as possible, while the longer focus lens nearer the 

 stage secures a larger disc than would be possible 

 with a lens of shorter focus. The convex sides of 

 both the lenses are turned towards each other. 

 When working with the \ inch and higher powers, 

 a further concentration of the light is effected by an 

 achromatic condenser. If the edge of the flame is 

 turned towards the condensers, the light is very 

 bright, but there is a difficulty in illuminating a 

 large field uniformly. If, however, the flame make 

 an angle of about 5 degrees with the optic axis, this 

 difficulty disappears without sensibly reducing the 

 brightness of the field. 



The arrangement in which an optical lantern 

 replaces the simple lamp may be used with the 

 advantageous results of shortening exposures, and 

 facilitating focussing under high powers. 



When working at night, there is no necessity to 

 use a focussing cloth. The image is first received 

 on a screen of the finest possible ground-glass, made 

 more transparent by waxing the ground surface, but 

 even this is too coarse for delicate focussing, and to 

 get the final focus the screen is removed and 

 replaced by a lens of about 1 inch focus held in the 

 hand, but attached to a strip of wood, whose arms 

 are brought up against the end of the camera when 

 in use, so that, in shifting it about to examine 

 different parts of the image, it is constrained to 

 remain in one plane, and its distance has been so 

 adjusted once and for all, by previously focussing 

 the ground-glass when in position, that its focal 

 plane is exactly that occupied by the sensitive plate. 

 Mr. Stanley's arrangement for focussing consists of 



