ILLUMINATING THE OBJECTS. 161 



small diaphragm : at night, from the concentrated light of 

 a sperm-oil or gas lamp, having a faint blue-tinted chim- 

 ney-glass, to correct the yellow colour which predominates 

 in all our artificial illumination. If in the day-time, close 

 a portion of the window-shutters. 



" Sixthly. In all cases when high powers are used, the 

 intensity of the illumination should be increased by optical 

 contrivances below the object and stage : this is generally 

 effected by using achromatic condensers beneath the stage. 

 The apparatus for illumination should be as perfect as the 

 magnifying power." 



If these directions are strictly followed, no injury to the 

 eyes from using a microscope need be feared. 



Mr. Ross very properly remarks, that the manner in 

 which an object is lighted is second in importance only to 

 the excellence of the glass through which it is seen. When 

 investigating any new or unknown specimen, it should be 

 viewed in turns by every description of light, direct and 

 oblique, as a transparent object and as an opaque object, 

 with strong and with faint light, with large angular pencils 

 thrown in all possible directions. Every change will pro- 

 bably develope 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 ; the last having a refrac- 

 tive power nearly equal to that of glass. 



If the object be a delicate vegetable tissue, it will be, in 

 some respects, rendered more visible by gently heating or 

 scorching before a clear fire, between two plates of glass. 

 In this way the spiral vessels of asparagus and other 

 similar vegetables will be beautifully displayed. Dyeing 

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

 woods, will, in some cases, answer the purpose better. 



But the principal question in regard to illumination is 

 the magnitude of the illuminating pencil, particularly in 

 reference to transparent objects. Generally speaking, the 

 illuminating pencil should be not quite so large as can 

 be received by the lens : any light beyond this produces 

 indistinctness and glare. The superfluous light from the 



M 



