66 THE MICROSCOPE, 



the object-glass is turned small enough to pass through the aperture 

 in the centre of the Lieberkuhn ; but in the lower powers, where a 

 great amount of reflecting surface would be lost on account of the large 

 size of the glasses employed if this plan were adopted, the aperture 

 in the centre of the Lieberkuhn is made to admit as many rays as 

 will fill the field of view, and no more. 



Mr. S. Highley's achromatic gas-lamp, fig. 44, is now much in use, 

 for condensing the light on the mirror. Gas, as a source of light, 

 presents great advantages over oil and spirit, on account of clean- 

 liness, being ever ready for use, and affording a perfect control over 



fig. 44. 



the flame ; but when the ordinary gas-lamps are used for the purpose 

 of illuminating the field of the microscope, a yellow glaring light is 

 given, alike injurious to the eye and the definition of the object under 

 examination. To correct these evils this lamp was arranged, which is 

 also otherwise useful to the microscopist. It consists of a stage A, 

 supported by a tube and socket, sliding on an upright rod rising from 



