THE CHRISTIAN PHILOSOPHER. 



No. Vin. p. 118. On Telescopes ; with a brief 

 notice of a NEW REFLECTING TELESCOPE, 

 constructed hy the author. 



It is doubtful to what particular individual 

 we owe the invention of the telescope. Some 

 nave supposed that Roger Bacon and Baptista 

 Porta invented this instrument. Borelli ascribes 

 the invention to Zacharias Jansen, a native of 

 Middleburgh. Perhaps the account given in the 

 article to which this note refers, and which is 

 stated by a variety of authors, may be as proba 

 ble as any other. It is certain that the telescope 

 was not in general use until the beginning of 

 the 17th century, and that no discoveries in the 

 heavens were made with it, till the year 1609. 



There are two kinds of telescopes, refracting 

 and reflecting. In refracting telescopes, the rays 

 of light pass through convex or concave glasses 

 or lenses. The object-glass is always convex, 

 and forms an image or picture of the object in an 

 inverted position in its focus ; which image is 

 viewed by the eye-glass ; and the magnifying 

 power is in the proportion of the focal distance of 

 the object-glass to that of the eye-glass. The 

 focal distance of a convex glass may be ascer 

 tained by holding it in the rays of the sun, op 

 posite to a piece of white paper, and measuring 

 the distance between the glass and the white 

 spot, or burning point, formed on the paper. An 

 astronomical telescope for viewing celestial ob 

 jects may be constructed with only two glasses. 

 If an object-glass, 30 inches focal distance, be 

 fixed in the end of a tube, and an eye-glass of 

 one inch focus be placed at. the other end, at the 

 distance of 31 inches from the object-glass, a 

 telescope will be formed, which will magnify in 

 the proportion of one to thirty, or 30 times ; that 

 is, objects seen through such a telescope will ap 

 pear thirty times larger in diameter, or thirty 

 times nearer, than to the naked eye. By such 

 an instrument, the inequalities on the moon s 

 surface, and some of the satellites of Jupiter, 

 may be perceived ; but when directed to land ob 

 jects they will appear inverted, or turned upside 

 down. In order to reverse the appearance of 

 the object, two other eye-glasses are required ; 

 or, if a concave eye-glass of a similar focus be 

 placed at 29 inches from the object-glass, the 

 object will appear in its natural position, and the 

 magnifying power will be the same ; but the 

 field of view will be much smaller. Astro 

 nomical telescopes of this construction were for 

 merly made of 120, and even of 200 feet in 

 length, and were used without a tube ; the ob 

 ject-glass being placed on the top of along pole ; 

 but these are now entirely superseded by achro 

 matic telescopes. In the achromatic telescope, 

 the object-glass is compounded of two, and 

 sometimes of three lenses, placed close to each 

 other, one of which is a double concave of 

 white flint glass, and the other a double convex 



of crown glass. By this means an image is 

 formed without being blended with .the prismatic 

 colours ; and it will, therefore, bear a much 

 greater magr jfying power than a common refrac 

 tor. An achromatic telescope four feet long 

 will magnify objects as much as a common re 

 fractor 100 feet long. 



In reflecting telescopes the images of objects 

 are formed by speculums or mirrors, instead of 

 lenses. They are of two kinds, the Gregorian 

 and the Newtonian. The Gregorian reflector 

 consists of a tube, in which a concave mirror, 

 having a hole in its centre, is placed. The rays 

 of light from distant objects falling upon this 

 mirror, form an image before it, in its centre or 

 focus. This image is intercepted by a smaller 

 mirror, which reflects it back through the hole in 

 the large mirror, to an eye-glass, through which 

 the observer views the object. In the Newtonian 

 reflector, a plane mirror, placed at an angle of 

 45 degrees, is substituted in place of the small 

 mirror in the Gregorian construction, and the 

 observer looks down upon the object through the 

 side of the tube. Dr. Brewster has suggested 

 an interesting improvement in the construction 

 of this instrument, which is described in the 

 Edinburgh Ency. Art. Optics, p. 644. 



New Reflector. About three years ago, the 

 author commenced a series of experiments on 

 reflecting telescopes ; and has lately constructed 

 several on a new plan and principle. In this 

 construction, there is no small speculum, either 

 plane, convex, or concave ; there is no tube, ex 

 cept a short one of two or three inches in length, 

 for holding the speculum. The observer sits with 

 his back to the object, and views the image formed 

 by the speculum through an eye-piece, which re 

 quires to be nicely directed and adjusted. Three 

 or four instruments of this construction have been 

 fitted up, with specula of 5, 8, 16, 28, 35, and 

 49 inches focal distance. One of them, having 

 a speculum of eight inches focus, and two inches 

 diameter, with a terrestrial eye-piece, magnify 

 ing about 25 times, forms an excellent parlour 

 telescope for viewing land objects, and exhibits 

 them in a brilliant and novel aspect. When 

 compared with a Gregorian of the same size 

 and magnifying power, the quantity of light upon 

 the object appears nearly doubled, and the image 

 is equalty distinct. It represents objects in their 

 natural colours, without that dingy and yellowish 

 tinge which apnears when looking through a 

 Gregorian. Another of these instruments, hav 

 ing a speculum of 28 inches focal distance, and 

 an eye-piece producing a magnifying power of 

 about 100 times, serves as an excellent astrono 

 mical telescope. By this instrument the belts 

 and satellites of Jupiter, the ring of Saturn, and 

 the mountains and cavities of the moon, may be 

 contemplated with great ease and distinctness. 

 By placing the pedestal on the four of the apart* 



