Mercury Globules as Test Objects. By J. W. Gordon. 17 



glance at fig. 6, which illustrates the optical system, shows that 

 the conditions of illumination are identical over an appreciable 

 range of distance along the optical axis in the region occupied by 

 the speculum globule (gi). It is also clear from this consideration 

 that the light reflected from the surface of the speculum globule 

 does not fill the whole aperture of the Microscope, but passes along 

 certain zones, these zones being more central when the globule is 

 near the eye-lens, and more peripheral when it is remote from the 

 eye-lens. A very pretty experiment can be made by moving the 

 speculum globule slowly from one of its extreme positions to the 

 other. We can then watch the gradual change in the appearance 

 of the imaG;e as it is transmitted through different zones of the 

 system. The most noticeable change is that the colour varies, the 

 image being, as a rule, strongly blue at one end and distinctly red 

 at the other, a good achromatic image being obtained at some 

 intermediate point. 



The diagram, fig. 6, shows the path of an incident pencil from 



OBJECTIVE. 



OCULAR 



Pig. 6. 



a point on the object globule in full lines — the path of a reflected 

 pencil in broken lines. It is obvious that the diameter of the 

 black disk seen in the object globule (^/ 2 ) can be very easily 

 calculated. Looking down the instrument we have in the field 

 the original of the picture pourtrayed in the object globule, and it 

 is seen under the full magnifying power of the Microscope. It 

 exhibits, of course, a bright field, an illuminated edge of the 

 globule, which melts into the field, and a dark centre, the diameter 

 of which last depends upon the aperture of the objective and the 

 angle at which the light from the condenser strikes the under face 

 of the globule. If this latter factor were known it would be an 

 easy thing to calculate the diameter of the darkened part of the 

 disk by the formula of equation (1), but as the exact angle of the 

 condenser cone depends upon the focusing of the condenser, 

 and as, moreover, the focusing of the condenser may most 

 conveniently be adjusted with reference to the brightness of the 

 resulting image, this cannot very well be made the subject of 

 calculation. But since it is to be seen in the Microscope and of 

 full size it can quite easily be made the subject of measure- 



Feb. 19th, 1908 c 



