xxii INTRODUCTION. 



reflected by the parabolic surface upon the plane reflector, and thence upon the upper 

 surface of the object. This apparatus has lately given place to the next-mentioned. 



JBeck'ts Opaque Illuminator, which is a valuable piece of apparatus, is constructed thus : 

 A short screw-tube, or adapter, with an aperture in one side> is fitted between the end of 

 the body and the top of the object-glass. Within the tube is a circle of thin glass, set 

 obliquely, so that the light entering the side aperture is reflected by the circle upon the 

 surface of the object, and passes upwards to the eyepiece. This may be used with the 

 fa to T^-inch immersion-lenses; and with it the hexagonal markings of Plenrosiyma 

 anyulatum are distinctly seen, and the lines of fracture running through the areoles 

 (Moorehouse). 



Tolks's Illuminator consists of a prism inserted in the side of the object-glass, between 

 the front and middle combinations, so reflecting the light entering by a side aperture upon 

 the object. See also ILLUMINATION. 



A number of points in regard to the colour of objects, distinction of pigment-granules 

 from minute air-bubbles, &c. may be decided by these pieces of apparatus. In questions 

 of elevations or depressions of surface, conclusions must be based upon the analysis of the 

 formation and arrangement of the shadows, and not upon the general appearance, because 

 it is well known that objects, or parts of them, usually appear larger and more prominent 

 in proportion to the amount of light reflected by them to the eye. Hence, for instance, 

 little depressions, which are in fact extensions of surface, by reflecting more light than the 

 surrounding flat or nearly flat surfaces, would appear very brilliant and luminous, and thus 

 resemble elevations. 



Camera Lucida, and steel di^k or Mirror of Scunnering. One of these is requisite for 

 drawing from the microscope. The camera lucida resembles that commonly used in 

 sketching landscapes &c., but is provided with a fitting adapting it to the eyepiece. The 

 mirror of Soniniering is a plane mirror of polished steel, less in diameter than the pupil of 

 the eye, supported opposite the focus of the eyepiece by a small steel arm attached to a 

 split ring which grasps the eyepiece by a spring-action. There is one disadvantage 

 attending the eyepiece of Sommeriug, viz. that it inverts the image of objects, which the 

 camera does not. When either of these is used, the body of the microscope must be placed 

 horizontally, and the axis of vision be directed perpendicularly ; the image of the object 

 will then be seen upon the table, and may be traced with a pencil. In using the camera, 

 it must be remembered that the size of the object will appear greater as the distance 

 between the eyepiece and the table is increased ; hence it is best always to place the 

 microscope in one and the same position when about to use it for drawing, so that the 

 extent to which the objects are magnified by the same power may always be the same. 

 The pin mentioned at page xv is invaluable for this purpose. By placing a inicroineter- 

 slide upon the stage, and comparing the magnified image of the divisions with those on a 

 known measure, such as a graduated rule, the magnifying power can always be checked, 

 and any error arising from varied distance determined. 



Beale's neutral-tint glass reflector is often used as a camera, and is inexpensive. 



In using either the camera or the mirror of Sb'mmering, the eye must be kept exactly in 

 one position ; otherwise the image of the object will move. Also the field and the paper 

 must be illuminated to nearly the same extent. One of the screens mentioned at page xxviii 

 is very useful for excluding extraneous light. 



Erecting-ylms (Lister's). This consists of a brass tube, furnished with a meniscus at 



