lxix 



Perhaps one of the most marvelous facts connected with this phenome- 

 non is, that while these ethereal billows possess such wonderful dynamic 

 energy that their concentrated impulses can excite the atoms of metals into 

 such violent clashing as to effect a dissolution of their compact masses, 

 they dash harmlessly upon the far more delicate reticulations of the optic 

 nerve, simply because their periods of vibration do not correspond with 

 any of those of the atoms of the nerve tissues. 



Doctor Tyndall concentrated the rays from an electric light by a glass 

 lens, and in this beam he interposed, in a glass vessel, a solution of iodine 

 and bisulphide of carbon by which every ray of light was cut off. The 

 heat rays, however, which constituted nine- tenths of the whole beam, 

 passed freely through, and in the dark focus thin plates of tin and zinc 

 were readily fused and brown paper set on fire. Into this focus he fear- 

 lessly placed his eye. The heat upon the eyelids was unbearable, and to 

 protect them from it he cut a circular aperture through a card slightly 

 larger than the pupil, and through this opening these powerful rays were 

 actually thrown upon his retina without the least harm. A removal of 

 the iodine would have permitted the concentrated rays of light to have 

 entered the pupil, and the instant destruction of the retina, by its atoms 

 being roused into excessive vibration, would have been the result 

 "Nothing," says this eminent phi'osopher, "could more forcibly illus- 

 rate the special relationship which subsists between the optic nerve and 

 the oscillating periods of luminous bodies. The nerve, like a musical 

 string, responds to the periods with which it is in accordance, while it 

 refuses to be excited by others of vastly greater energy which are not in 

 unison with its own." 



I have endeavored to give you, very briefly, an idea of the theory 

 adopted by the most eminent men of the present day, as that which clearly 

 explains the phenomena of light and heat. I have also tried to give you 

 some account of a very few of the wonderful properties possessed by the 

 invisible rays which outline each end of the prismatic colors of the solar 

 spectrum. I will now briefly refer to some of the marvels discovered, 

 by the spectroscope in the variously tinted field comprising the visible por- 

 tion of the spectrum. 



Dr. Wollaston was, I believe, the first to discover that the colors 

 were not continuous from one end of the spectrum to the other, but 

 that the continuity of the tints was interrupted by dark vertical bands 

 which seemingly severed them at several places in the different colors. 

 These dark bands soon attracted the attention of many scientific ob- 

 servers. Frauenhofer devoted so much labor to the mapping of them, 

 and to their description, that they are known as Fraunhofer's lines. In 

 the spectrum their respective positions are invariably the same. 



Let us imagine a small circular saw, laid flatwise, and having only six or 

 eight large teeth projecting at intervals from its periphery, and upon these 

 teeth as many prisms placed with their sides vertical and their angles cor- 

 responding with the angles of the teeth of the saw, and we will then have 

 the chief feature in the arrangement of a spectroscope. 



