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ohscura, there would be an image of the object formed at the 

 other end. That image would be in proportion to the focal 

 length of the glass, so that the telescope which had the 

 longest focal distance would magnify the best. It is the 

 area or surface of the glass, or the metal, which gives the 

 quantity of light, but a speculum of one-inch size will pro- 

 duce the same magnifying power if the focal length is fifty- 

 four feet, as the six feet telescope of Lord Rosse. The only 

 other thing with reference to the principle was the examination 

 of the object, which can be shown by a simple experiment 

 with a lantern or candle. If, for instance, he placed a mag- 

 nifying glass near a candle or a lamp, he would have thrown 

 on a piece of paper the image of the candle inverted. The 

 forms of telescopes are very various, and the Earl of Rosse 

 has greatly improved the previous practice with reference to 

 the construction of telescopes. One of the most striking 

 things was his very fine invention for the casting of a speculum 

 of large magnitude. The substance of which the speculum 

 is made was a mixture of tin and copper, thus forming an 

 alloy of the most brittle substance known; hence the difficulty 

 of casting speculums so that they would bear the abrasion 

 of polishing and grinding. Not long since he met with an 

 Irish gentleman at the Earl of Rosse's residence, who had 

 received a gold medal from the Society of Arts, for the 

 completeness with which he had constructed a speculum of 

 nine inches in diameter. That gentleman told him that he 

 had made thirty different castings, and out of these he had 

 obtained only two perfect specimens. The Earl of Rosse 

 had cast four speculums of six feet in diameter instead of 

 nine inches, and weighing three tons instead of fifteen or 

 twenty pounds; out of these he got three perfect castings, 

 one of which, however, was unfortunately afterwards spoiled 

 by the carelessness of a workman. Connected with the pre- 

 paration of the metal there is a matter of interest to the 



