128 SUMMARY OF CURRENT RESEARCHES RELATING TO 



clamping rod are placed as far away from the optical centre as possible 

 in order to prevent any disturbance of the focus from expansion when 

 subjected to a strong heat-producing light. 



" The substage consists of a tube about 3 inches long with a short- 

 focus condenser at the proximal end and the diaphragm plate at the 

 distal end. It can be moved either backwards or forwards or can be 

 accurately centered by screws which are shown in the woodcut. This 

 particular form of substage possesses. Dr. Kibbler considers, manifold 

 advantages. In the first place the diaphragm plate, being removed to 

 some little distance from the stage and having the short-focus condenser 

 in front of it, is thrown quite out of focus with the object-glass and 

 consequently does not tend in any way to diminish the area of the field, 

 but, on the contrary, produces a general and uniform diminution of light. 

 But what is of still more importance the diaphragm-plate is found in 

 this combination to have developed new functions and acts somewhat 

 similarly to the " stop " iised by photographers in the photographic 

 lens. That is, it increases both the area of definition and the depth of 

 focus. Without the condenser the diaphragm-plate, to produce a 

 similar efiect, would have to be removed to a distance that would 

 l»ecome inconvenient in practice. The condenser obviates this by 

 projecting the diaphragm-plate optically further away by making it 

 still more out of focus and so lessens the distance at which it is 

 necessary to be placed. The condenser also has the effect of converting 

 what otherwise would be a straight pencil of light, into a cone before it 

 reaches the object and transforms it into a more suitable form of illumi- 

 nation for showing the defining powers of an object-glass to the best 

 advantage." 



The instrument is made entirely of brass and possesses great 

 stability. 



Mawson and Swan's Photomicrographic Apparatus.— This ap- 

 paratus (fig. 21) is of an extremely simple character and enables an 

 ordinary camera to be used for photomicrography. 



It consists of a light metal disc, which can be screwed on the camera 



front in place of the ordinary 

 ^^^' ^■^' -.,==^ lens, the opening in the centre 



being furnished with the Society 

 screw, so that ordinary micro- 

 scopic objectives can be readily 

 attached. Upon three horizontal 

 rods projecting from this disc 

 slides another similar disc, also 

 with an opening in the centre, 

 and having a pair of small spring 

 clips for the slide which it is 

 desired to photograph. The 

 third rod is encircled, behind 

 the stage, by a spiral spring, and focusing is effected by turning the 

 screw-nut on the rod, which forces the stage towards the objective, the 

 spring moving it back again when the screw is released. 



Robinson's Photomicrographic Cameras. — Messrs. J. Eobinsou 

 & Sons make two forms of cameras which are of an extremely simj)]e 

 character. 



