NATURAL PHILOSOPHY. 139 



distance travelled by a particle between consecutive collisions is about 

 44T7fU() f an i ncn > tne mean velocity being about 1,50-3 feet per second; 

 and therefore each particle makes 8,077,200,000 collisions per second.. 4. 

 The laws of the diffusion of gases, as established by Professor Graham, Jr., 

 are deduced from this theory, and the absolute rate of diffusion through an 

 opening can be calculated. The author intends to apply his mathematical 

 methods to the explanation on this hypothesis of the propagation of sound, 

 and expects some light on the mysterious question of the absolute number 

 of such particles in a given mass. 



COLOR-BLIXDNESS. 



If there is one infirmity or defect of those five senses with which we are 

 mo.st of us blest which more than any other attracts sympathy and claims 

 compassionate consideration, it is blindness, an inability to know what is 

 beautiful in form or in color, to appreciate light, or to recognize and compre- 

 hend the varying features of our fellow-men, a perpetual darkness in the 

 midst of a world of light, a total exclusion from the readiest, pleasantest, 

 and most available means of acquiring ideas. 



And yet who would suppose that there exists, and is tolerably common, a 

 partial blindness, which has hardly been described as a defect for more than 

 half a century, and of which it may be said, even now, that most of those who 

 suffer from it are not only themselves ignorant of the fact, but those about 

 them can hardly be induced to believe it. The unhappy victims of this par- 

 tial blindness (which is real and physical, not moral) are at great pains in 

 learning what to them are minute distinctions of tint, although tp the rest of 

 the world they are differences of color of the most marked kind, and, after 

 all, they only obtain the credit of unusual stupidity or careless inattention, 

 in reward for their exertions and in sympathy for their visual defect. We 

 allude to a peculiarity of vision which first attracted notice in the case of 

 the celebrated propoundcr of the atomic theory in chemistry, the late Dr. 

 Dalton, of Manchester, Avho, on endeavoring to find some object to compare 

 in color with his scarlet robe of doctor of laws, when at Cambridge, could 

 hit on nothing which better agreed with it than the foliage of the adjacent 

 trees, and who, to match his drab coat, for our learned doctor was of the 

 Society of Friends, might possibly have selected crimson continuations, as 

 the quietest and nearest match the pattern-book of his tailor exhibited. 



An explanation of this curious defect will be worth listening to, the more 

 so as one of our most eminent philosophers, Sir John Herschel, has recently 

 made a few remarks on the subject, directing attention at the same time to 

 other little known but not unimportant phenomena of color, which bear 

 upon and help to explain it. 



It is known that white light consists of the admixture of colored rays in 

 certain proportions, and that the beautiful prismatic colors seen in the rain- 

 bow are produced by the different degree in which the various rays of color 

 are bent when passing from one transparent substance into another of 

 different density. Thus, when a small group of color-rays, forming a single 

 pencil or beam of white sunlight, passes into and through the atmosphere 

 during a partial shower, and falls on a drop of rain, it is first bent aside on 

 entering the drop, then reflected from the inside surface at the back of the 

 drop, and ultimately emerges 4n an opposite direction to its original one. 

 During these changes, however, although all the color-rays forming the 



