BOSS'S ACHROMATIC ILLUMINATOR. 61 



connected to a pinion ; and .by means of a rack attached, the inner 

 tube, carrying the achromatic combination d, is raised or lowered : the 

 upper part of the outer tube is larger than 

 that where the milled-head is seen, for the ^M 



purpose of allowing the milled ridge of y 



the achromatic to pass up and down. For 

 the i or ^ inch, the combination d is used ; 

 and for the higher power, ^ or -jL, 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 ascertained by turning aside the mi- 

 croscope tube, and observing when the spot of light appears on the 

 object placed on the stage. The microscope-tube is then to be re- 

 placed as nearly over the spot of light as possible, and vision of the 

 object obtained, disregarding the precise quality of the light. Then 

 proceed for perfect adjustment as directed in using Gillett's con- 

 denser. 



Slight obliquity of the illumination subdues the glare attendant 

 upon perfectly central and full illumination by lamp-light; and this 

 obliquity may be obtained by slightly altering the position of the mir- 

 ror; or if the mirror is not employed, but light is obtained by pointing 

 the microscope-tube directly to the lamp, then the obliquity required 

 may be obtained by a small variation of the inclination of the micro- 

 scope. It is essential that the mirror and lamp-glass be free from dust 

 and soot. 



F. H. Wenham, Esq., (Micros. Trans. 1851) proposed a new illu- 

 minator, for the purpose of obtaining perfect definition under high 

 powers. Those who have experimented on the subject, may have ob- 

 served that there is something in the nature of oblique light reflected 

 from a metallic surface particularly favourable for the purpose of bring- 

 ing out minute markings, which may, in some measure, be attributed 

 to the circumstance of light so reflected being purely achromatic. In 

 order to render this property available, Mr. Wenham contrived a very 

 ingenious metallic reflector, by which the condensation of lateral light 

 may be effected. 



