gillett's illuminator. 155 



be adjusted by its sliding-tube (without disturbing the 

 microscope-tube) until the images of the diaphragms in 

 the object-glass and centering lens are distinctly seen. 

 The illuminator should now be moved by means of the 

 left-hand screw on the secondary stage, while looking 

 through the microscope, to enable the observer to recog- 

 nise the diaphragm belonging to the illuminator, and by 

 means of the two adjusting screws, to place this diaphragm 

 central with the others; thus, the first condition, that of 

 centricity, will be accomplished. Remove the white paper 

 from the mirror, and also the centering-glass, and replace 

 the cap on the eve-piece, also the object on the stage, of 

 which distinct vision should then be obtained by the rack 

 and pinion, or fine screw adjustment, should it have 

 become deranged. 



The second process is to ascertain that the fittest con- 

 centration of light is obtained. For this purpose the 

 mirror should now be so inclined that the image of some 

 intercepting distant object, such as a house-top, or chim- 

 ney, tree, window-frame, or (if lamp-light be employed) 

 the lamp's flame may be brought into the field of view; 

 these, though not distinctly seen, may be recognised by 

 partially darkening or otherwise occupying the field; then 

 distinct vision of such object must be obtained by means 

 of the rack and pinion moving the secondary stage to and 

 from the object. Excepting the case of the lamp's flame, 

 the above objects are considered as the representatives of 

 the source of light ; for when daylight is employed — as, for 

 example, a white cloud — its motion prevents the image 

 being easily produced: then it is convenient to employ a 

 distant object, such as the above, — the difference of the 

 focal length of the illuminating lens for such an object, 

 and for the white cloud, being almost insensible. This 

 last adjustment being effected by the movement of the 

 secondary stage alone, the microscope tube remaining un- 

 disturbed, also the object on the object-stage uninterrupted 

 in focus, the source of the illuminating light and the 

 object to be examined will both be distinctly seen at the 

 same time. These adjustments, whether for daylight or 

 lamplight, being completed, the mirror may be turned so 

 as wholly to reflect the light either of the sky or of the 



