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and if the diameter of the clear aperture of the object-glass be divided by the diameter of this miniature image, the quotient gives the exact magnifying power of the eye-piece, when used with that particular object-glass. By the aid of the Dynamometer the diameter of the miniature image can be accurately measured, and the power of the eye-piece thence ascertained.

Having arranged the Telescope as above described, the process of measuring the image is performed as follows :—Holding the Dynamometer near the eye-piece, observe the miniature disc by the aid of an ordinary pocket lens of low power, shifting the Dynamometer until the two internal edges exactly touch the circumference of the image, note the division opposite the point of contact; this is the diameter of the image in decimals of an inch. Dividing now the diameter of the clear aperture of the object-glass by this decimal, the quotient is the exact magnifying power of the eye-piece when used with that particular object-glass. Example :—The clear aperture of the object-glass being 6*24 inches, and the diameter of the image, as indicated by the Dynamometer, being ’026, we obtain, by dividing 6-24 by ’026, a quotient of 240, which is the magnifying power.

The diameter of wire, sheet metal, or other suitable object, may be accurately ascertained by sliding it along until the two edges just touch the internal sides of the Dynamometer, when the division exactly opposite the point of contact is equal to the diameter of the article measured. Thus, if a wire passes freely up the wide end of the triangular opening until it is stopped at the first short division beyond '04, its diameter is *042 of an inch. The scale is so divided that each long division represents -oi, or the hundredth part of an inch. Each of the first two long divisions from o is divided into 10 parts, each of which is equal to '001, or the thousandth part of an inch. Each of the remaining long divisions is divided into five parts, each equal to ’002, or the two-thousandth part of an inch.

THE ASTRONOMICAL SPECTROSCOPE.

It is not the writer’s intention to go into the theory of the spectroscope, as so many admirable works have been written on this subject, but to briefly notice its application to the telescope. The student should consult Schellen’s, Roscoe’s, or Lockyer’s books of the subject, or a most useful little book just issued by Proctor, should he desire further information. The spectroscope has lately been extensively used in the investigation and study of the heavenly bodies, and with wonderful success, fresh discoveries constantly being made.

Two forms of spectroscopes are commonly employed for astrono^ mical purposes. The first consists of one or more prisms, with their apices pointing in the same direction, each prism widening the spectrum received from that next to it. As such a series is limited, on

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