32 THE MICROSCOPIST. 



Fig. 6 shows Collins's Harley binocular microscope, a 

 good second class instrument. 



Fig. 7 represents Beck's large compound miscroscope 

 (monocular) ; and Fig. 8, Hartnack's small model micro- 

 scope, with the body made to incline. - 



Fig. 9, Cachet's inverted microscope, invented by Dr. 

 Lawrence Smith for chemical investigations. 



CHAPTER III. 



MICROSCOPIC ACCESSORIES. 



IN addition to the object-glasses, eye-glasses, mirror, 

 and mechanical arrangement of the microscope, to which 

 reference was made in the last chapter, several accessory 

 instruments will be useful and even necessary for certain 

 investigations. 



The Diaphragm, for cutting off extraneous light when 

 viewing transparent objects, is generally needed. In some 

 German instruments it consists of a cylinder or tube, whose 

 upper' end is fitted with a series of disks having central 

 openings of different sizes. The disk can be adjusted to 

 variable distances from the object on the stage so as to 

 vary its effects. English and American opticians prefer 

 the rotary diaphragm, which is of circular form, perforated 

 with holes of different sizes, and made to revolve under 

 the stage. The gradual reduction of light can be accom- 

 plished by the cylinder diaphragm, since when it is pushed 

 up so as to be near the stage it cuts off only a small part 

 of the cone of rays sent upwards by the concave mirror, 

 but, when drawn downwards, it cuts off more. 



Collins's Graduating Diaphragm, which is made with 

 four shutters, moving simultaneously by acting on a lever 



