ON THE ILLUMINATION OF OBJECTS. 191 



light and back again, to ascertain if the spot of light passes 

 across the centre of the field of vision; if not, it may be 

 corrected by turning the mUled-head screw of the illuminating 

 apparatus which presses against the front edge of the stage, 

 and when, by adjustment, it is found that the spot of light 

 passes centrally across the field, then, having adjusted the 

 microscope tube so as to bring the spot of light to the centre 

 of the field of vision, screw the microscope tube fast in its 

 place. To effect the second adjustment mentioned above, the 

 mirror must be moved about until an image, more or less 

 distinct, of some adjacent object — such as a window-bar or 

 chimney-pot — crosses the field of the microscope ; the most 

 distinct vision of this object is then to be obtained by turning 

 the milled-head screw on the side of the illuminating appa- 

 ratus, the microscope tube remaining stationary with the object 

 in its focus. When these general adjustments are accom- 

 plished, the whole may require correction, which must be 

 effected by a repetition of the process. The mirror may then 

 be turned up to the sky, that being the source of light, and 

 its best state for illuminating microscopic objects is by means 

 of white clouds opposite the sun. The utmost perfection of 

 vision is generally obtained by this adjustment for day-light 

 illumination upon delicate objects ; but by lamp-light these 

 objects are sometimes seen best by placing the achromatic 

 illuminating lens a little nearer the object than the distance 

 which produces a distinct image of the source of light, at the 

 same time that the object is in focus. Objects having some 

 little thickness are best seen when the lens is a little farther 

 off than the distance just mentioned. This last position of 

 the illuminating lens diffuses the light more equally over the 

 field. 



" When lamp-light is used, an object-glass of lower power 

 than that which it is intended to employ for observation, 

 should be applied to the microscope tube, and the adjustment 

 performed as before directed. Attention must be paid to this 

 additional circumstance, namely, that the image of the lamp 

 be placed in the middle of the illuminated disc of light, by 

 means of the mirror ; the mirror and lamp are then to remain 



