70 



ON THE GENERAL DIFFUSION OF KNOWLEDGE. 



Fig. 1, 



Fig. 2. 



Masons, bricklayers and others, who are in 

 terested in this subject, would do well to pro 

 cure and study Count Rumford s &quot; Essay,&quot; 

 which was originally sold for two shillings. 

 His directions have seldom been accurately 

 attended to in this country by those who have 

 pretended to improve chimneys on the princi 

 ples he has laid down, partly from carelessness, 

 and partly from ignorance of the elements of 

 science. When the grate is not set in its proper 

 place, when its sloping iron back is retained, 

 when no pains have been taken to make its ends 

 coincide with the covings of the fire-place, 

 when the mantle, instead of having its back 

 rounded off , is a vertical plane of iron, cutting a 

 column of smoke which rises beneath it ; and, 

 above all, when the throat of the chimney, in 

 stead of four, is made, as we often see, fourteen 

 inches wide, not one of the Count s directions 

 has been attended to, and his principles have as 

 little to do with the construction of such a chim 

 ney as with the building of the dykes o&amp;lt;&quot; Hol 

 land, or the pyramids of Egypt. 



A knowledge of the science of Optics, which 

 explains the nature of vision, and the laws by 

 which light is refracted and reflected, is essen 

 tially requisite to the makers of telescopes, mi 

 croscopes, and all other dioptric and catoptric 

 instruments, in order to carry them forward to 

 meir highest pitch of improvement. And yet 

 how often do we find many of those employed 

 in the construction and manufacture of such in 

 struments glaringly deficient in the first princi 

 pals of optical science? One maker imitates 

 the instruments of another without discrimina 

 tion, and while he sometimes imitates the ex 

 cellencies, he as frequently copies the defects. 

 Hence the glaring deficiencies in the construc- 

 ioft of the eye-pieces of most of our pocket te 



lescopes, and the narrow field of view oy wliicu 

 they are distinguished, which a slight acquaint 

 ance v/ith the properties of lenses would teach 

 them to obviate. By a moderate acquaintance 

 with the principles of this science, any inge 

 nious mechanic might, at a small expense, be 

 enabled to construct for himself many of those 

 optical instruments by which the beauties of the 

 animal and vegetable kingdoms, and the won 

 ders of distant worlds have been explored. 



Although, in the hands of mathematicians, 

 the science of optics has assumed somewhat of 

 a forbidding appearance to the untutored mind, 

 by the apparently complex and intricate dia 

 grams by which its doctrines have been illus 

 trated, yet it requires only the knowledge of a few 

 simple facts and principles to guide an intelli 

 gent mechanic in his experiments, arid in the 

 construction of its instruments. In order to the 

 construction of a refracting telescope, it is only 

 requisite to know, that the rays of light, passing 

 through a convex-glass, paint an image of any 

 object directly before it, at a certain point be 

 hind it, called its focus ; and that this image 

 may be viewed and magnified by another convex- 

 glass, placed at a certain distance behind it. 

 Thus, let CD, fig. 1, r-epresent a convex-glass, 

 whose focal distance CE is 12 inches ; let AB 

 represent a distant object directly opposite ; the 

 rays of light passing from this object, and cross 

 ing each other, will form an image of the object 

 AB, at EF, in an inverted position. Let GH 

 represent another convex-glass, whose focal 

 distance is only one inch. If this glass is placed 

 at one inch distant from the image EF, or 13 

 Fig. 1. 



