186 THE POPULAR SCIENCE MONTHLY. 



to make the diffraction disks obtrusively large is to point the telescope 

 at a rough stone building in very strong sunlight. The small crystal- 

 line surfaces in the stone reflect the sun in little shining points of light, 

 which, observed through the telescope, make the building appear as if 

 stuck all over with silver dollars, while an unnatural glassy blurring 

 of the whole image is very apparent. If the illumination will bear it, 

 this appearance can be greatly exaggerated by covering the object- 

 glass with a pasteboard diaphragm in such a manner as to considerably 

 reduce its clear aperture. 



For exactly the same reason, a similar blurred appearance is dis- 

 agreeably noticeable when objects like the Moon or Jupiter are ob- 

 served with an extremely high power. 



From what has just been said, it is obvious that a power higher 

 than that due to a one-sixth-of-an-inch eye-piece is of very little use 

 in connection with an object-glass whose focal length is about thirteen 

 times its clear aperture ; but, had the waves of light been created more 

 minute than they are, it would have been possible to employ with 

 advantage a still higher power. 



It is thus seen that the focal lengths of telescopic eye-pieces, no 

 matter what the size of the object-glass may be, should all lie between 

 the very narrow limits of two and a half inches for the lowest power 

 and one sixth of an inch for the highest power ; six or seven of them 

 give a suflicient range of magnifying power to fully utilize the object- 

 glass of any telescope. 



A convenient way of expressing the limiting magnifying powers 

 of a telescope in terms of the size of its object-glass, independently of 

 its ratio of aperture to focal length, is easily deduced from the above 

 by a simple proportion, and is as follows : a telescope will not bear 

 with advantage a lower magnifying power than five nor a higher 

 magnifying power than seventy-five for every inch of clear aperture 

 of its object-glass. 



In all that has gone before, we have confined ourselves to the con- 

 sideration of the single set of light-waves originated by a single vi- 

 brating molecule, and to single-convex lenses, having surfaces of the 

 proper curvature, to convert the convex spherical or plane wave-fronts 

 into concave spherical wave-fronts ; but how is it in reality ? 



We have seen that the light of the sun originates in clouds of pre- 

 cipitated carbon from the great upward currents of metallic vapors 

 rising from its interior. It can be demonstrated that the molecules of 

 water are so small that, were one drop enlarged to the size of the 

 earth, the individual molecule would only come up to the size of horse- 

 chestnuts. I'liere is no reason to think that carbon-molecules differ 

 greatly from this in size. Therefore we receive from the sun the enor- 

 mous number of light-waves originated by each vibrating molecule, 

 suspended through a depth of many miles in the transparent vapors at 

 the surface of a globe 885,000 miles in diameter. These light-waves 



