428 Transactions of the Society. 



the effective source of light. It is a very convenient light source, 

 because its brilliancy can be varied within wide limits, and very 

 simply. By varying the distance between the filament and the 

 ground-glass surface, the luminosity of the latter can be rapidly 

 and greatly altered, since it varies inversely with the square of the 

 distance between the filament and the ground-glass film. The light 

 which in this way enters the glass speculum is transmitted almost 

 intact by total internal reflection along the length of the glass rod. 

 If a glass be chosen which has low absorbent power and is free 

 from optical defects, the illumination is almost constant at all 

 points of the speculum. The optical system of such a speculum 

 presents some very interesting features, but its discussion would 

 demand more space than can be allotted to it in this paper. It 

 may suffice here to say that these internal reflections give rise to a 

 figure in which a central disk of light is seen surrounded by a 

 succession of luminous rings concentric with it, the illumination 

 falling off gradually towards the edges of the pattern. The 

 diameter of the central disk and of its concentric rings is, of course, 

 determined primarily by the diameter of the rod. Its appearance, 

 however, is dependent equally upon the magnifying power under 

 which it is viewed. A rod of about \ in. diameter yields a disk 

 of very serviceable size. 



The glass speculum, which is conveniently made about 6 in. 

 in length, but may be of any required dimensions, is mounted in a 

 carrier which holds it in a horizontal position, and is provided at 

 the end opposite to the ground-glass already described with a 

 polished surface, flat or lenticular in form, according to the user's 

 requirements. The use of a lens, when the end of the rod is shaped 

 to a lenticular form, is to focus the condenser short of the ground- 

 glass at some point in the rod where the speculum pattern is of a 

 convenient size. By speculum pattern I mean the appearance of 

 the light source when some plane in the interior of the speculum 

 is chosen as the source of light. The speculum pattern has the 

 same general character of a luminous central disk surrounded by 

 bright rings, as the ground-glass seen with the aid of the speculum. 

 But in the speculum pattern itself there is no visible grain of the 

 ground-glass. This results from the circumstance that the light 

 at any point within the speculum is supplied by rays coming from 

 various points upon the ground-glass. The features of various 

 points of origin therefore are combined, with the result that the 

 luminous patch is as structureless as is the flame of an oil lamp. 

 We have, in fact, a diffusion similar to that which results from 

 throwing the ground-glass out of focus ; but the use of a cylindrical 

 reflector secures at the same time that there shall be no corre- 

 sponding loss of light or loss of angle, since the light which would, 

 if unreflected, have become diffused over a broad wave-front, is 

 here condensed by reflection into the original and unvarying area. 



