ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



767 



similar stage of ground opal glass of equal thickness (fig. 166). After four 

 years' experience the arrangement is found to answer well. 



Fig. 1GG. 



(.3) Illuminating- and other Apparatus. 



Mercury Vapour Lamp for Microscopical Work.* — In this mercury 

 vapour lamp (fig. 167) designed by J. E.Barnard, the illumination is 

 obtained from an exhausted tube partially filled with mercury, the 

 passage of the current through which renders the vapour luminous. 

 The light emitted is confined to a few wave-lengths widely separated, 

 which permits of the use of monochromatic light, by the aid of screens, 

 in several regions of the spectrum, thus affording good contrast with 

 different stains. Even without screens the light is more efficient than 

 other forms of illumination giving a continuous spectrum, there being 

 no red rays emitted. The size and shape of the tube permit of its use 

 for critical illumination. 



The tube A, with resistance B, which can be supplied to suit any 

 voltage from 80 to 250, is inclosed in a metal cover 0, 16 X 7*5 X 

 12*5 cm. (6£ in. X 3 in. x 5 in.), mounted on a heavy base with 

 square upright, adjustable for height and tilt for starting the lamp, 

 which is simply effected by pressing up the lever E, thus cutting out a 

 portion of the resistance and depressing the left side, until that portion 

 of the tube is filled with mercury ; on slowly bringing the tube back to 

 the horizontal, still keeping the lever E pressed up, an arc will be formed, 



* C. Baker's Special Catalogue, 1908. 



