242 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
he is able to adjust the direction of the beam of light so as to be central, in which case 
it must go directly to the distant reflectors whose images lie close together, not x^th 
of an inch apart, on the teeth of the wheel. 
Various optical difficulties presented themselves, and the arrangements were altered 
at various times. These chiefly consisted in illumination of the field of view, and were 
successively removed. Four improvements specially deserve notice:— 
1. A general and intense illumination of the whole field was soon traced to a 
reflection from the centre of the object-glass. To obviate this a circle of black velvet 
about one inch in diameter was fastened to the centre of the object-glass on the inside. 
This was quite successful. 
2. In using powerful lights an intense blaze was reflected from the toothed wheel, 
which made delicate observations impossible. We first tried smoking the toothed 
wheel, but not only did it still reflect a good deal of light but the regularity of form of 
the teeth was impaired ; finally we used a highly-polished toothed wheel slightly 
bevelled, and by tilting the revolving mechanism the light was reflected to the upper 
parts of the interior of the adapter, which being blackened absorbed the light. This 
arrangement succeeded admirably, but some experiments were also made with the 
smoked wheel. 
3. The field still had some general illumination owing to reflection from grains of 
dust, or scratches, or other imperfections in the plane-glass diagonal reflector. We 
objected to the glass reflector also for another reason : at an angle of 45° a double 
layer of glass reflects only 0T6 of the incident light ; on the return of the light from 
the distant reflector 0’84 of this quantity passes through to the eye of the observer. 
The whole light obtained from this arrangement is then 0T6X0 , 84=0G36; but if 
we could have an arrangement to reflect 0*5 of the light, so that on its return 
we should see 0*5X0 *5 = 0*2 5 of the light, we should nearly double the intensity. 
We accomplished this by substituting for the diagonal glass reflector a silvered reflector 
with an oval hole whose projection on a plane perpendicular to the axis of the 
telescope is a circle, and whose dimensions are such as to allow one-half of the light 
to pass through. The diagonal reflector is three inches from the toothed wheel. This 
arrangement certainly doubled the light, and owing to the darkness of the field its 
superiority over the glass reflector was enormous. 
4. To prevent the slight illumination still remaining on the toothed wheel from 
causing inconvenience to the observer, a strip of metal with a series of holes of varying 
size was sometimes placed in the secondary focus of the eye-piece. It could be easily 
removed and replaced, so that when everything was prepared without its aid this 
diaphragm was inserted, using so small a hole as to show only the distant reflectors 
and two teeth of the wheel. 
The toothed wheel *—In contriving the revolving toothed wheel and the mechanism 
* The term “ the toothed wheel ” applies in this description to that toothed wheel in the mechanism 
between the teeth of which the light is made to pass. 
