MICROSCOPIC ILLUSTRATIONS. 255 



1 1 . When daylight is used vision is much improved 

 by the use of diaphragms: these, however, are inert, 

 unless the image of them in the visual pencil is less than 

 that of the aperture of the object-glass? or magnifier, with 

 which they are used : for example, if we put a diaphragm 

 one-twentieth of an inch in diameter in the focus of a one- 

 twentieth of an inch lens, having also one-twentieth of an 

 inch of aperture, or, of course, in one of shorter focus, 

 because the aperture of the lens, when viewed by a magni- 

 fier, will be found illuminated throughout its whole extent, 

 just as if no diaphragm was used ; but, when it is placed 

 out of the plane of the focus, say half, or three-fourths of 

 an inch off, then its effect will be very decided. In this 

 case, it reduces the intensity of the illumination, and pre- 

 vents faint objects from being drowned in an excess of 

 light, while the darkness and strength of all the markings 

 of the object are decidedly increased ; it moreover seems 

 to improve the oblique pencils of any instrument, and 

 renders spherical aberration, if not very strong, almost in- 

 sensible. In fact, the quantity of light in the visual pencil 

 is reduced exactly in the same manner as if the aperture 

 of the object-glass or magnifier was cut off to the standard 

 of the image of the diaphragm seen in it. 



12. There is no modification of daylight illumination 

 superior to that invented by Dr. Wollaston, provided it is 

 used in a proper manner, whether the object is of the 

 lined or proof kind, or not. In order to obtain an effective 

 oblique light with it, it is necessary that the axis of the 

 optical part of the instrument should not be in the same 

 right line with the centre of the perforation, but deviating 

 considerably from it, whicji gives oblique light in a line 

 drawn from the centre of the aperture of the lens, or 



