xx INTRODUCTION. 



behind the first pair of lenses of the condenser, and screwed into the top of the condenser 

 in the place of the ordinary diaphragm. Neither of these kinds of stop equals in conve- 

 nience the improved Gillett's condenser, because with the latter the number of rays 

 transmitted or intercepted, and the degree of their obliquity, can be varied by the simple 

 rotation of the diaphragms. The spot-lens is also used for the same purpose. This consists 

 of a very convex plano-convex lens, placed beneath the stage, the central rays being inter- 

 cepted by a stop. 



The central stop is generally used when objects are examined with the higher powers. 

 The power used in the condenser will vary greatly according to the kind of object under 

 examination. If a considerable amount of light be required without obliquity of the rays, 

 the condensing power should be lower than that of the object-glass. If great obliquity 

 of the rays bo required, the higher the power of the condensing lenses, and the larger 

 their angular aperture, the better. When the achromatic condenser is suitably arranged 

 in regard to centering, and the condensing object-glass or set of lenses is properly selected 

 and adjusted, the structure of minute objects is displayed in a manner with which those 

 who regard the condenser as useless must be utterly unacquainted. Many very delicate 

 objects are rendered most distinct by using the smallest aperture in the diaphragm of the 

 condenser, so that the admitted rays are almost parallel. But this requires a very 

 powerful light. 



"Extra Eyepieces. Always one, and mostly two or more eyepieces, or oculars as they 

 are sometimes called, are obtained with the microscope when purchased : but the highest 

 eyepiece which is made should always be procured j for although high eyepieces are so 

 far objectionable that they magnify the imperfections of the image formed by the 

 object-glass as well as the image itself, yet they frequently render parts of structure 

 distinct which are perhaps only just perceptible with a lower eyepiece. Kellner's ortho- 

 scopic eyepiece, in which the lower lens is doubly convex, gives a very large and flat 

 field. 



Polarizing Apparatus or Polariscope. This usually consists either of two plates of 

 tourmaline, or of two Nicol's prisms. The latter are generally used, and are preferable 

 on account of their freedom from colour. They are composed each of two half-rhombs of 

 calcareous spar cemented together so as transmit only one image. The prisms should 

 appear perfectly clear and colourless, and free from scratches and veins ; and when, on 

 holding them to a light, the uppermost is rotated so as to occupy a particular position 

 with regard to the other, no light should be transmitted through them. 



The polarizing apparatus is useful in bringing to light certain peculiarities of structure 

 which cannot be detected in any other way ; and is particularly useful in the study of 

 minerals (RocKs). A substitute may be made of two crystals of the iodo-disulphate of 

 quinine, dried upon and cemented to circles of thin glass. In use, one is placed beneath 

 the object, and the other on the top of the eyepiece. 



Spectroscope. The spectroscope, as applied to the microscope, is a somewhat expensive 

 apparatus, and requires great practice in its application. It is, however, most important 

 in many investigations. See SPECTROSCOPE. 



Side Condenser. This consists of a large doubly convex or plano-convex lens, or " bull's- 

 eye," of short focus, 2 or 3 inches, mounted upon a brass arm, which slides up and down a 

 rod placed perpendicularly in a stand. The arm should be capable of being lengthened, and 

 the stand should be so broad and heavy that there need be no fear of its being overturned. 



