ILLUMINATION BY TRANSMITTED LIGHT. 



differently-coloured rays would proceed from nearly equally bright 

 points of the source of light (e.g., a bright sky), and therefore, 

 produced backwards, meet it, under these circumstances an 

 achromatic system would be preferable. Since, however, such 

 restrictions seldom occur in practice when the instrument is 

 provided with a concave mirror, and may certainly be avoided in 

 most cases, the manufacture of achromatic illuminating devices 

 appears to be unnecessary. 



From what has been pointed out, the different illuminating 

 devices are therefore effective in two directions only : they give 

 to the cone of light, which illuminates the field of view, an equal 

 intensity throughout, and, in the second place, extend its angular 

 aperture. The statements with regard to any other influence they 

 possess are purely imaginary ; there is no foundation for such 

 assertions as, for instance, that they cause the disappearance of 

 the interference lines at the edge of the object, and resolve finer 

 details better in proportion to their freedom from aberration, &c. 

 With the same diaphragm and equal focal length, an ordinary 

 condensing lens is just as effective as the most complex system of 

 lenses, provided the diameters of the refracting surfaces and of 

 the mirror correspond, for all colours, to the size of the dia- 

 phragm. On this assumption the angle of aperture of the 

 incident cone will be the greater, the nearer the diaphragm to 

 the object-plane. It is quite immaterial whether the focal point 

 of the illuminating apparatus lies in the object-plane or not; 

 its position is practically important only when, on close approxi- 

 mation to the objective, the marginal rays, produced backwards, 

 no longer meet the mirror surface, and consequently the maxi- 

 mum of brightness is nearly reached when the image formed of 

 distant objects by the illuminating apparatus appears in the field 

 of view. 



If the illuminating apparatus is to satisfy all requirements, it 

 must also be provided with means, besides the diaphragm, applied 

 above the last refracting surface, to shut out any desired portion 

 of the cone. Many objects appear with the greatest distinctness if 

 the central rays are cut off, whilst others appear more distinct if 

 illuminated by oblique light incident on one side only. To facilitate 

 the illumination, in addition to the usual graduated series of 

 diaphragms, central stops are needed, which may be conveniently 

 applied on a disc rotating over the diaphragm. It is advisable, on 



