ROSS'S ACHROMATIC ILLUMINATOR. 77 



matic combinations is supplied, adapted for the illumina- 

 tion of the whole range of the powers of the microscope : 

 the whole syotem being employed for the highest powers; 

 two of such combinations with the middle powers ; and the 

 largest combination 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 ob- 

 ject-glass, as an illuminating 

 lens, must be screwed on to 

 it. In fig. 43, two tubes are 

 seen sliding one within the 

 other; to the outer one, 6, is ' 

 attached a flat plate a, which 

 slides underneath the stage, 

 and is adjusted for distance by 

 the screw /; at c the milled- 

 head is connected to a pinion; 

 and by means of a rack at- 

 tached, the inner tube, carry- 

 ing the achromatic combina- Fig ' -*'' condenser. 

 tion dj is raised or lowered : the upper part of the outer 

 tube is larger than that where the milled-head is seen, for 

 the purpose of allowing the milled ridge of the achromatic 

 to pass up and down. For the ^ or inch, the combina- 

 tion d is used; and for the higher power, 1 or -J^, the 

 second e is slipped over d. Place the object to be viewed 

 upon the stage of the microscope; and when the instru- 

 ment is not directed at once 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 ascer- 

 tained 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 



