646 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
work draw-tube in those early days was intended solely for the purpose 
of obtaining large variations in power apart from changing the objective ; 
now, however, it is only used for the purpose of correcting the objective, 
i.e. as a substitute for collar correction. 
Baker’s D.P.H. Microscope No. 1. — This instrument (fig. 151) has 
diagonal rack-and-pinion coarse movement, micrometer screw, and lever 
fine adjustment, giving a movement of 1 /225 of an inch for each revolu- 
tion of the milled head ; draw-tube, every 10 mm. of which is engraved 
Fig. 151. 
with a ring, extending from 150 mm. to 250 mm., thus allowing the use 
of either English or Continental objectives ; mechanical stage giving a 
movement of 25 mm. in either direction, graduated to 1/2 mm., the 
milled heads being below the level of the top plate, so that large culture 
plates can be used ; the distance from optic axis to limb (2J in.) allow- 
ing of their easy manipulation. The stage has stops to enable either a 
3 in. by 1 in. or a 3 in. by 1J in. slip to be brought always to the same 
place for the purpose of recording any given field ; but these, together 
with the stage clips, are removable. 
