AMERICAN INSTITUTE. 



689 



our eyes are aifected by undulations, recurring four hundred and 

 fifty millions of times in a second. And if yellow, five hundred 

 millions of vibrations must occur before we can appreciate it. 

 To discriminate violet, seven hundred millions of movements are 

 communicated to the fibrilla of the retina. A white substance on 

 a black ground 1,400th of an inch square, may be distinguished 

 with the naked eye, without certainty. But particles which reflect 

 light, such, for example, as gold dust, of the minuteness of a 

 millionth part of an inch, may be plainly discerned with the 

 naked eye. If there is one minute particle in a picture that can- 

 not be discerned, make a short row of them and the eye will 

 distinguish them immediately; showing that the delicacy of 

 human vision is much greater for lines than single particles. 



When you enter a picture gallery, you imagine that you behold 

 all the pictures at once, not being conscious of the motions of 

 the eye; when, the fact is, each picture is successively presented 

 to it with the rapidity of lightning. If the eye were steady, 

 objects would disappear. Try the experiment; fix your eye on 

 a single picture, which is a hard thing to do, owing to its dispo- 

 sition to motion, and you will find that it will first become obscure 

 and then vanish. The reason is, the retina of the eye is subject 

 to fatigue by the colors, lights and shades of objects striking 

 upon the same parts, and thus exhausting the nerve, which actu- 

 ally requires constant exercise. The eye catches red more rapidly 

 than any other color, as is instanced in battle; twelve soldiers 

 wearing red will be shot, when only seven wearing green, six 

 brown, five Austrian gray. All photographic effects are, no 

 doubt, the effects of a very high temperature. Whenever a ray 

 of light impinges on any point, it raises its temperature to the 

 same degree as the source from whence it springs. If it were 

 not so, the images would not be sharp on their edges. 



On the 7th of May, 1857, Mr. Bond, the eminent astronomer 

 of Boston, established by successful experiments, that stars down 

 to the fifth magnitude, or in other words, " all stars usually visi- 

 ble to the naked eye, may be mapped by the aid of photography 

 with a degree of accuracy unsurpassed by the most refined meas- 

 urements; this was proved by measuring the distances of the pho- 

 [Am. Inst.] 44 



