78 



ON THE GENERAL DIFFUSION OF KNOWLEDGE. 



tion. It is not denied, that several inventions 

 have originated in this way, but they are much 

 fewer than is generally imagined; and, in al 

 most every instance, where chance suggested 

 the first hint of any invention, the future im 

 provements were directed by the hand of ge 

 nius and the aids of science. It is said, that the 

 invention of the telescope was owing to a spec 

 tacle-maker s boy having accidentally taken up 

 two convex glasses of different focal distances, 

 and placed the one near his eye and the other 



at a considerable distance, when he perceived, 

 on looking through them, the spire of a neigh 

 bouring church turned upside down, and much 

 larger than its usual size. The father of the 

 boy, amazed at this singular appearance, be 

 thought himself of adjusting two glasses on a 

 board, supporting them in two brass or wooden 

 circles, which might be removed nearer to, or 

 farther from each other at pleasure, as in the 

 following figure, where A represents the object, 

 B the lens next the object, a the inverted image 



formed by it, C the glass next the eye, and D 

 the sliding board on which it was fixed, for ad 

 justing the focus. Such appears to have been 

 the first rude construction of a telescope. But 

 KO long as the invention remained in this state, 

 it was of little benefit to society. It was not 

 before Galileo, a philosopher of Tuscany, heard 

 of the circumstance, and entered into investiga 

 tions on the refraction of light, and the proper 

 ties of lenses, that this noble instrument was 

 improved and directed to the heavens for the 

 purpose of making astronomical discoveries ; 

 and all the subsequent improvements it has re 

 ceived, have been the result of reasonings, and 

 experimental investigations, conducted by men 

 of science. Sir Isaac Newton, inconsequence 

 of his experiments and discoveries respecting light 

 and colours, detected the true cause of the im 

 perfection of the common refracting telescope, 

 and suggested the substitution of metalline spe 

 cula instead of lenses, which led him to the in 

 vention of the reflecting telescope ; and Mr. 

 Dollond, in consequence of his investigations and 

 experiments respecting the different degrees of re 

 fraction and divergency of colour produced by 

 different kinds of glass, effected the greatest im 

 provement that had ever been made on the re 

 fracting telescope, by producing an image free 

 of the imperfections caused by the blending of 

 the prismatic colours. And we have reason to 

 blieve, that the further improvement of this tele 



scope will chiefly depend on ascertaining the 

 true chymical composition of flint gla^s for achro 

 matic purposes, and the proper mode of conduct 

 ing its manufacture, which may lead to the con 

 struction of instruments of this kind, on a more 

 extensive scale than has ever yet been attempted, 

 and to discoveries in the celestial regions far 

 beyond those which have hitherto been made. 

 But such improvements can never be effected, 

 unless by numerous experimental investigations, 

 conducted by those whose minds are thoroughly 

 imbued with the principles of chymical and op 

 tical science.* 



One of the latest improvements on Achromatic 

 object-glasses was made by a foreigner of the name 

 of GvJnand, who was originally a cabinet-maker. 

 After acquiring a knowledge of the principles of 

 optics, and of the mode of constructing telescopes, 

 he applied himself particularly to ascertain the 

 proper composition of JHnt-gttiss for achromatic 

 purposes; and, after spending twenty or thirty 

 years in making experiments casting one pot of 

 glass after another, and meeting with frequent 

 disappointments, lie at length succeeded in obtain 

 ing glass for achromatic telescopes, of larger dimen 

 sions and of a quality superior to what could for 

 merly be procured. Of this glass was formed the 

 largest triple achromatic telescope ever constructed, 

 which was lately erected in the observatory of the 

 university at Dorpat, under the direction of M 

 Fraunhofer. This glass is perfectly free from veins, 

 and has a greater dispersive power than any ob 

 tained before. The diameter of this object glass is 

 almost ten inches, and its focal distance 15 feet. 

 It has four eye-pieces, the lowest magnifying 175 

 times, and the highest 708 times. Mr. Tulley ot 

 Islington lately constructed, of similar material*, 



