Ill 
hand. The rod is supported at the eye-piece end of the 
telescope, on a hearing attached to a metal collar made to clip 
the telescope by a screw, and attached to this bearing is a 
ratchet wheel, which prevents the too rapid reverse motion 
of the india-rubber diaphragm. The ratchet is not absolutely 
necessary with the present apparatus, owing to the friction of 
the pinion against the wheel, preventing the india-rubber 
contracting too rapidly. 
It is not considered necessary to give the dimensions of the 
parts of the apparatus, as in every case these must vary with 
the diameter of the object-glass of the telescope. The prin- 
cipal point to be attended to, is the width of the broader ring, 
which is covered inside by the india-rubber, and this must 
be sufficiently wide to close the aperture when in use. In 
order to prevent the over- winding of the diaphragm, stops are 
placed at each end of the teeth on the half wheel. 
An index of some kind will be necessary to make the 
apparatus complete. A pointer might be fixed on the end of 
the dew-cap, and a scale might project from the wheel. The 
diameter of the various openings could also be determined by 
the ear, the number of teeth in the wheel and pinion being 
known, the ratchet wheel could be so adapted, that a certain 
number of clicks would indicate that a certain aperture is 
then in use. This method, however, might be tedious to 
the observer, owing to the necessity for counting. The index 
and scale will perhaps be more convenient, and if the light 
from the sky should not be sufficient to show the reading of 
the scale, the micrometer lamp could be placed so as to illu- 
minate the scale and index only. 
The additional weight at j^ie end of the telescope renders a 
counterpoise necessary, and this is effected on my telescope 
by a slight addition to the counterpoise already attached. 
It may perhaps be necessary to say, that I have as yet tried 
the apparatus merely for the mechanical working, but am 
very sanguine that it will answer the purpose intended more 
effectively, than any other method I am acquainted with. 
