190 USE or THE MICROSCOPE. 



lowest in power to that which is intended to be attached to 

 the microscope ; so that when the one-eighth is used on the 

 microscope, the one-fourth is screwed into the illuminating 

 apparatus, and so, in like manner, with the rest. But when 

 economy is not regarded, a system of three achromatic com- 

 binations is supplied, adapted for the illumination of the 

 whole range of the powers of the microscope, the whole 

 system being employed for the highest powers, two of such 

 combinations with the middle powers, and the largest combi-. 

 nation by itself, for the lowest powers. This illumination is 

 not required for objects when viewed with object-glasses 

 transmitting small pencils of rays, or whose angular aperture 

 is less than thirty degrees — that is, where the object-glass is 

 of greater focal length than half an inch. 



" The apparatus is fixed to the under side of the stage of the 

 microscope, in the place of the diaphragm plate ; and before 

 fixing, the proper object-glass, as an illuminating lens, must 

 be screwed on to it. Place the object to be viewed upon the 

 stage of the microscope, and when the instrument is not 

 directed at once (as after noticed) to the source of light, such 

 as the flame of a lamp or a white cloud, arrange the reflector 

 (having the plane mirror upwards) so as to throw the light up 

 the tube of the apparatus ; which may be ascertained by 

 turning aside the microscope tube, and observing when the 

 spot of light appears on the object placed on the stage. The 

 microscope tube is then to be replaced, as nearly over the 

 spot of light as possible, and vision of the object obtained, 

 disregarding the precise quality of the light. 



" The two following important adjustments must next be 

 effected: — first, make the image of the source of hght (as 

 flame of lamp or light cloud) distinctly seen at the same time 

 that the object on the stage is seen; and, secondly, make 

 the axis of the tube of the illuminating apparatus coincide 

 with that of the tube of the microscope. The fine adjustment 

 is accomplished by turning the milled-head screw on the side 

 of the tube of the apparatus, until the brightest illumination 

 is seen through the microscope; then move the microscope 

 tube from its central position sideways across the spot of 



