56 THE MICROSCOPE. 



all possible directions. Every change will probably develop some new 

 fact in reference to the structure of the object, which should itself be 

 varied in the mode of mounting in every possible way. 



It should be seen both wet and dry, and immersed in fluids of 

 various qualities and densities ; such as water, alcohol, oil, and Canada 

 balsam; which last has a refractive power nearly equal to that of glass. 

 If the object be delicate vegetable tissue, it will be, in some re- 

 spects, rendered more visible by gentle heating or scorching before a 

 clear fire, between two plates of glass. In this way the spiral vessels 

 of asparagus and other similar vegetables may be beautifully displayed. 

 Dyeing the objects in tincture of iodine, or some one of the dye-woods, 

 will, in some cases, answer this purpose better. 



But the principal question in regard to illumination is the mag- 

 nitude of the illuminating pencil, particularly in reference to trans- 

 parent objects. Generally speaking, the illuminating pencil should be 

 as large as can be received by the lens, and no larger. Any light be- 

 yond this produces indistinctness and glare. The superfluous light 

 from the mirror may be cut off by a screen, having various-sized aper- 

 tures placed below the stage. 



The DiapJiragm, fig. 38, is the instrument used for effecting this 

 purpose. It consists of two plates of brass, one of which is perforated 

 with four or five holes of different sizes ; this plate is of a circular 



figure, and is made to revolve 

 upon another plate by a cen- 

 tral pin or axis ; this last 

 plate is also provided with 

 a hole as large as the largest 

 in the diaphragm-plate, and 

 corresponds in situation to 

 the axi of the compound 

 body. To ascertain when 



either of the holes m the diaphragm-plate is in the centre, a bent 

 spring is fitted into the second: plate, and rubs against th edge of 

 the diaphragm-plate, which is provided with notches. The space be- 

 tween the smallest and largest is great enough to use for the purpose 

 of shutting off all the light from the mirror. 



A good mode of imitating artificially the light of a white cloud 

 opposite the sun has been proposed by Mr. Varley : he covers the sur- 

 face of the mirror under the stage with carbonate of soda, or any similar 

 material, and then concentrates the sun's light upon its surface by a 

 large condensing lens. 



