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most, for each inch of aperture. That is to say, with a three-inch telescope we should gain no advantage by using a power much above one hundred and fifty, and certainly none above three hundred.

But a large telescope also has its defects, owing to the impossibility of bringing all the light to absolutely the same focus. There is a limit to the magnification which can be used, rather difficult to define exactly, but of which the observer will be very sensible when he looks into the instrument and sees the blue aureole already mentioned.

But there is still another trouble, which annoys the astronomer more than all others, but which the public rarely understands.

We see a heavenly body through a thickness of atmosphere which, were it all compressed to the density that it has around us, would be equal to about six miles. We know that when we look at a body six miles away, we see its outlines softened and blurred. This is mainly because the atmosphere through which the rays have to pass is constantly in motion, thus producing an irregular refraction which makes the body look wavy and tremulous. The softened and blurred effect thus produced is magnified in a telescope as many times as the object itself. The result is that as we increase the magnifying power we increase a certain indistinctness in the vision in the same proportion. The amount of this indistinctness depends very much on the condition of the air. The astronomer having this in mind tries to find a perfectly clear air, or, rather, air which is very