PRACTICAL MICROSCOPY. 



REFRACTIVE INDICES. 



Air ................ I'ooo 



Water ................ -336 



Sea water .............. -343 



Alcohol .............. -373 



Glacial acetic acid .......... '380 



Equal parts, glycerine and water . . . . ' 400 



Glycerine .............. '475 



Oil of turpentine ............ 478 



Crown glass ............ I 5 to 525 



Homogeneous-immersion fluid ...... 500 



Chloride of cadmium in glycerine . . . . ' 500 



Cedar- wood oil ............ '512 



Canada balsam ............ -532 



Flint glass .............. -575 



Monobromide of naphthaline ...... '658 



Bisulphide of carbon .......... '678 



Oil of anise .............. i'8n 



Sulphur .............. 2*115 



Phosphorus .............. 2-224 



Plane or flat mirrors reflect an image of the same size as 

 the object, the flame of a gas or oil lamp for instance, and 

 therefore the rays are parallel, and the image is not inverted. 

 When light is reflected from glass, the under side of which 

 is silvered, much of it is lost from several causes ; but when 

 polished metal is employed for the reflecting surface, the 

 rays do not enter the substance of the reflector, and there 

 is less loss of light than in the former instance. 



We must now turn our attention to the concave mirror, 

 with which all respectable microscope stands are furnished. 

 In this kind, the focus is situated at a point at which the 

 reflected rays meet, and when rays parallel to the axis are 

 brought together after reflection, the meeting point or focus 

 is at an equal distance between the centre of curvature C 

 and the mirror itself, and, consequently, if a luminous object 

 be placed in this principal focus F, the rays emitted by the 

 whole surface of the mirror will be parallel, as seen in 

 Fig. 87. 



If, however, the luminous point be at a greater distance 



