xviii INTRODUCTION. 



scope when purchased ; but the highest eyepiece which is made should always be pro- 

 cured : for although high eyepieces are so far objectionable, that they magnify the imper- 

 fections of the image formed by the object-glass as well as the image itself, yet they fre- 

 quently render parts of structm-e distinct which are perhaps only just perceptible with a lower 

 eyepiece. 



Polarizing Apparatus. — This usually consists of either two plates of tourmaline, or of two 

 Nicol's prisms. The latter are generally used, and are preferable on account of their free- 

 dom from colour. They are composed each of two half-rhombs of calcareous spar cemented 

 together so as to transmit only one image. The prisms should appear perfectly clear and 

 colom'less, 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 hght 

 should be transmitted through them. 



The polarizing apparatus is useful in bringing to light certain peculiarities of structm'e 

 which cannot be detected in any other way, 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. 



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. 

 Its use is to condense the light upon opake objects. When used, it is placed between the 

 object lying upon the slide under the microscope and the lamp or other source of light, the 

 plane sm-face of the lens being at right angles to the direction of the rays of light ; and the 

 lens must be brought so close to the object that the focus falls upon the latter. Sometimes 

 a small condensing lens is used to concentrate the light abeady transmitted through the large 

 condenser ; this is usually fixed to some part of the microscope, but it is rarely required. 

 A doubly convex lens of much longer focus than the bull's-eye lens, about 7 or 8 inches, 

 will be found very useful for condensing the light upon the mirror when the achromatic con- 

 denser, stops, &c. are used with the highest powers. It should be placed veiy near the lamp. 

 The arm of the bull's-eye lens may be adapted to hold either or both the lenses. 



Amici's prism is sometimes useful for throwing very oblique light through a transparent 

 object. It consists of a very flat triangular glass prism, the two narrower sides of which are 

 convex. The third and broadest side forms the reflecting surface. The prism may be 

 attached to a separate stand, or to the stand of the microscope. It exerts a condensing as 

 well as reflecting action. 



Lieberkuhn. — Some opake objects may be well illuminated by a lieberkuhn or silver 

 cup ; by which the light, first reflected by the mirror upon the concave surface of the cup, 

 is afterwards reflected upon the object. But this can only be used for small objects, and is 

 therefore not generally useful. 



The discovery of the importance of excluding the central rays of light and using a central 

 stop for this purpose is due to Mr. F. H. Wenham, who has invented an appai-atus in 

 which this principle is taken advantage of. It is known as 



Wenham' s Parabolic Reflector, and consists of a brass tube fitted beneath the stage in the 

 place of the ordinary achromatic condenser, terminated above by a hollow truncated cone, 

 the perpendicular section of which forms a parabola, with an internal polished silver reflecting 

 surface. At the base of the parabola is placed a disk of thin glass, in the centre of which is 

 cemented a dark well. In use, the central rays are stopped by the dark well, whilst the 

 lateral rays passing up the tube impinge upon the parabolic surface, fiom which they are 

 reflected upon the lower surface of the object. This apparatus has since been modified by 



