254 I Assembly 



achromatic glass, by which all these difiiculties are obviated. 

 We have visual organs, by which we are able to see day light as 

 suet ; we see it white. And we can distinguish all the various 

 colors, the one from the other. We do this by a power resident 

 in man, through means of the optical instrument, perfect in its 

 construction, which the creator has given us. A glass which pos- 

 sesses the power of obviating this difficulty of the prismatic spec- 

 trum, is called achromatic. This is derived from two Greek words 

 a (signifying ivithout,) and chroma (signifying color. ) The word 

 microscope is also from two Greek words, meaning to view small 

 things. 9 



In 1773 a poor but well educated and enlightened English 

 gentleman, thought to experiment, in endeavoring to consauct an 

 optical instrument after the form of the human eye — for this is 

 the most perfect optical instrument of which Ave have any knowl- 

 edge. That it is one, may be seen at once by examining the eye 

 of an ox. You find imprinted upon the retina, which is the expan- 

 sion of the optic nerve, a beautiful picture of surrounding objects, 

 just such as are seen in the daguerreotypist's camera. (The Pro- 

 fessor her? drew a diagram of the human eye upon the board, and 

 showed the manner in which the rays of light fall upon and are 

 conveyed through it until they reach the retina, where they form the 

 picture already alluded to ; they pass through six media in going 

 from the outside to the retina.) This gentleman in carrying out 

 his idea, took common flint glass, and crown glass, their densities 

 and hence refracting power being differ *it, just as are the media 

 of the eye, and placed them together so- that they should fit accu- 

 rately the one to the other. . 



He took a double con^^ex lens of one material, and then fitted to 

 each side, a plano-convex lens of another material. So that he 

 had three media through which the rays must pass, and these 

 media fitted closelv, as thev are in the eve, making them as it were, 

 solid. This was the construction of the first object glass ever 

 made. 



In those days the wants of the naturalist were never thought 

 of; he must succumb to the astronomer. It was out of the ques- 

 tion to get any man to spend a moment of time, much less any 



