414 SUMMARY OF CURRENT RESEARCHES RELATING TO 



attempt to do away with the eye-piece micrometer ruled in squares. 

 Drs. Thomas and Gowers suggested engraving the lines on the base of 

 the cell itself, an eye-piece micrometer being replaced by an objective 

 micrometer. It is, however, in the first place, nearly impossible to 



Fig. 80. 



engrave lines as iiue as are required on such smooth and polished 

 glass as that of which the cell is made, so as to be clearly visible ; 

 there is also the risk of breakage, &c., and the inconvenience that 

 when the cell is filled with the liquid, the lines are still fainter and 

 unsuitable for being easily seen. 



The new arrangement consists of a metal plate C C, to which a 



Fig. 81. 



tube B, about 20 mm, h>ug, is screwed, containing on its upper part a 

 system of lenses, intended to form a very small image of a set of 

 divisions P in squares, engraved or photographed on glass, and j)laced 

 at the lower end of the tube. The tube is introduced into the opening 

 of the stage, and on the plate C C is jjlaced the cell of the hsemato- 

 meter, containing the liquid with the globules in suspension. These 

 soon fall to the bottom plate of the cell, and the focus of the lenses 

 being exactly upon this plate at 0, the image of the squares P is 

 formed there and is visible through the Microscope, at the same time 

 as that of the globules (see Fig. 82). 



