APPENDIX. 499 



which gives a cool, distinct, and definite view of the object. 

 When changing the object-glass on the compound body, the 

 examining eye-glass should always be employed to ascertain 

 that the central position of the condenser and microscope tube 

 is not deranged. When the condenser is used by artificial 

 light, the mirror must be removed, and the enamel disc, e, 

 applied by means of the bent arm. The campliine lamp is to 

 be placed upon the adjustable lamp stand, and the flame of the 

 lamp brought into the focus of the elliptical mirror. The 

 hole in the elliptical reflector, m, is to be brought in the optical 

 axis of the instrument, and the reflector directed so as to form 

 the image of the flame of the lamp upon the enamel disc. 

 The illuminating lens is then to be adjusted, until the surface 

 of the enamel disc is seen distinctly at the same time with the 

 object under examination, in the same manner as described 

 with reference to the tree or house-top, in the directions for 

 daylight, the surface of the mirror may be recognised by 

 making a small pencil mark upon it. In this apparatus, both 

 the intensity of the source of light, and the angle of the 

 illuminating pencil, are under command, the intensity being 

 regulated by the size of the image of the flame of the lamp, 

 produced upon the enamel disc by means of varying the place 

 of the lamp in the axis of the elliptical reflector, and the 

 angular dimension of the pencil, by means of the revolving 

 diaphragm plate. 



The examining glass mentioned in this article has been 

 arranged by Mr. Eoss, and is of the greatest service in center- 

 ing the compound body with the illuminating apparatus, of 

 whatever description it may be. It consists of a tubular cap, 

 similar to that which covers the eye-glass generally, but con- 

 tains a convex lens, which is so placed that the emergent 

 pencil at the eye-glass is in its focus. This emergent pencil 

 is the image of the aperture of the object-glass, also of the 

 apertures of the lenses and the diaphragms, which may be 

 contained in the condensing tube, and these are all sufficiently 

 near in focus at the same time for their concentricity to be 

 recognised. 



32* 



