at the distance answering to 33 yards 2-1/2 feet--which in this case exceeded the solar focus by an inch and a half. Then according to the above analogy, we have, as 43.5 : 42 :: 58.5 : 56.5 nearly. Hence the magnifying power due to the solar focal length of the telescope in question is 56.5, and the distance 33 yards 2-1/2 feet, is that which corresponds to an elongation of the solar focal distance an inch and a half.[31] If we multiply the terrestrial and the solar focal distances together, and divide the product by their difference, we shall again obtain the distance of the terrestrial object from the telescope. Thus, (43.5 + 42)/1.5 = 1218 inches = 101.5 feet, or 33 yards 2-1/2 feet. The magnifying power of a telescope is also determined, by measuring the image which the object-glass or the large speculum of a telescope forms at its solar focus. This is accomplished by means of an instrument called a _Dynameter_. This apparatus consists of a strip of mother-of-pearl, marked with equal divisions, from the 1/100th to the 1/1000th of an inch apart, according to the accuracy required. This measure is attached to a magnifying lens in its focus, in order to make the small divisions more apparent. When the power of a telescope is required, the person must measure the clear aperture of the object-glass, then holding the pearl _dynameter_ next the eye-glass, let him observe how many divisions the small circle of light occupies, when the instrument is directed to a bright object. Then by dividing the diameter of the object-glass by the diameter of this circle of light, the power will be obtained.[32] The most accurate instrument of this kind is the _Double Image Dynameter_ invented by Ramsden, and another on the same principle now made by Dollond, a particular description of which may be found in Dr. Pearson’s ‘Introduction to Practical Astronomy.’ The advantage attending these dynameters is that they do not require any knowledge of the thickness and focal lengths of any of the lenses employed in a telescope, nor yet of their number or relative positions; neither does it make any difference whether the construction be refracting or reflecting, direct or inverting. One operation includes the result arising from the most complicated construction. I shall only mention farther the following method of discovering the magnifying power, which is founded on the same general principle as alluded to above. Let the telescope be placed in such a position opposite the sun, that the rays of light may fall perpendicularly on the object-glass; and the pencil of rays may be received on a piece of paper, and its diameter measured. Then, as the diameter of the pencil of rays is to that of the object-glass, so is the magnifying power of the telescope. 8.--_On cleaning the lenses of telescopes._--