gillett's illuminator. 59 



eye-piece, a, is then to be adjusted by its sliding-tube (without dis- 

 turbing the microscope-tube) until the images of the diaphragms in 

 the object-glass and centering lens are distinctly seen. The illumi- 

 nator 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 recognise the diaphragm belonging to the illumuiator, 

 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 centeriug-glass, and replace the cap on the eye-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 be- 

 come deranged. 



The second process is to ascertain that the fittest concentration of 

 light is obtained. For this purpose the mirror should now be so in- 

 clined that the image of some intercepting distant object, such as a 

 house-top, or chimney, tree, window-frame, or (if lamp-light be em- 

 ployed) 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 second- 

 ary 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 illmninating lens for such an 

 object, and for the white cloud, being almost insensible. This last ad- 

 justment being effected by the movement of the secondary stage alone, 

 the microscope tube remaining undisturbed, also the object on the ob- 

 ject-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 lamp; and the eye-piece and object-glass 

 suitable for examining the object may be employed, and the focus ad- 

 justed accordingly. The conical diaphragm with its various apertures 

 may now be rotated, until that quality of illumination is obtained 



tube which holds the iUuminating combination, may all be in focus at the same time, 

 as with the same adjustment they may be brought sufSciently near in focus to recognise 

 their centricity. 



