1436 



VISION. 



are, indeed, very unsafe, unless the number of 

 facts observed is very great, and that is not 

 the present condition of our knowledge of the 

 copulative act. For that reason I have not 

 dwelt upon this, prima facie so obvious, line 

 of argument. 



Our knowledge of the function of the ve- 

 siculae seminales is, therefore, nearly in the 

 same condition as it was left by the great 

 Hunter, whose concise paper on this subject 

 is a master piece of reasoning and scientific 

 acumen. I have been able to add to this, be- 

 sides confirmation, little more than some few 

 observations tending to render it more pro- 

 bable that the secretion of the vesiculae is used 

 in copulation. I have, further, ventured on 

 an hypothesis which, 1 suppose, has suggested 

 itself to many before, and with which I am by 

 no means satisfied. So great, however, are 

 the difficulties surrounding the subject, that 

 in this unsatisfactory state the positive func- 

 tion still hypothetical I am compelled re- 

 luctantly to leave it. 



(S. R. Pittard.) 



VISION. ( Fr. vision, from Latin visio, from 

 video, visits, sight.) The faculty of seeing is one 

 of the chief means by which living creatures are 

 brought into relation with the world around, 

 and is the especial means by which they are 

 enabled to appreciate the wonderful pheno- 

 mena which flow directly and indirectly from 

 the creation of light. When in obedience to 

 the Divine command, " There was LIGHT," 

 there were organs created for its perception ; 

 and it is interesting to observe that the re- 

 storation of this gift of perception, when lost, 

 was among the most frequent, and certainly 

 not the least striking of the manifestations of 

 miraculous power displayed by the Saviour of 

 mankind. The vastness of the field over 

 which the faculty of vision gives us command, 

 the precision and permanence of this class 

 of our perceptions, the variety and accuracy 

 of the information it conveys, and the delight 

 it affords, lead us irresistibly to regard it as 

 the most perfect of our senses. In the in- 

 vestigation of this subject a train of minute 

 adaptation and wonderful contrivance is dis- 

 closed to us, in which are combined the ex* 

 treines of grandeur and of delicacy. There 

 is no department of science that possesses 

 a more absorbing interest than the laws of 

 optics when applied to the eye, and certainly 

 none which points with a steadier hand to the 

 wisdom of an omnipotent Creator. 



Very curious and unexpected information 

 respecting the early condition of the surface of 

 this planet and the ancient atmosphere has been 

 afforded by an investigation into the structure 

 of the organs of vision with which the earliest 

 marine animals were supplied. In the eloquent 

 language of Dr. Buckland, " with respect to 

 the waters wherein the Trilobites maintained 

 their existence throughout the entire period 

 of the transition formation, we conclude that 

 there could not have been that imaginary tur- 

 bid and chaotic fluid, from the precipitate of 

 which some geologists have supposed the ma- 



terials of the surface of the earth to be de- 

 rived : because the structure of the eyes of 

 these animals is such, that any kind of fluid 

 in which they could have been submerged at 

 the bottom must have been pure and trans- 

 parent enough to allow the passage of light to 

 organs of vision, the nature of which is so 

 fully disclosed by the state of perfection in 

 which they are preserved. With regard to 

 the atmosphere also, we may infer that had it 

 differed materially from its actual condition, 

 it might have so affected the rays of light, 

 that a corresponding difference from the eyes 

 of existing Crustaceans would have been found 

 in the organs on which the impressions of 

 such were then received. Regarding light it- 

 self, also, we learn, from the resemblance of 

 these most ancient organizations to existing 

 eyes, that the mutual relations of light to the 

 eye and of the e}'e to light, were the same at 

 the time when Crustaceans endowed with the 

 faculty of vision were first placed at the bot- 

 tom of primaeval seas as at the present mo- 

 ment." 



Light. To the opinions of the ancients on 

 the subject of light but little allusion need be 

 made, as they were but crude and vague con- 

 jectures. One, for instance, supposed that the 

 eyes emitted emanations of some sort by which 

 objects were, as it were, felt. Others ima- 

 gined that visible objects were constantly 

 throwing out from them spectral resemblances 

 of themselves, which, when received by the 

 eye, produced an impression of those objects ; 

 but in these fanciful notions there is little 

 satisfaction, and .we proceed at once to the 

 hypothesis of our illustrious countryman, Sir 

 Isaac Newton. According to his theory, 

 light was an imponderable substance, whose 

 inconceivably minute particles produced the 

 sensation of light by their action on the eye : 

 moving with immense velocity, they were 

 nevertheless acted on by attractive and repul- 

 sive forces residing in all material bodies, and 

 by these forces were liable either to be turned 

 aside from their natural straight course, re- 

 flected by the repulsive force, or penetrating 

 between the particles of bodies, to take a 

 direction on quitting them finally determined 

 by the position of the surface at which they 

 emerged. About the same time, however, a 

 very different hypothesis was advanced by 

 Huyghens, to the effect that all space is filled 

 with an extremely elastic and rare ether, and 

 that light is the result of the undulatory 

 movements communicated to this ether by 

 self-luminous bodies, which movements being 

 transmitted to the optic nerve, give rise to 

 the, sensation of light. The beautiful experi- 

 ments of Dr. Thomas Young strongly con- 

 firmed the truth of this theory, which is based 

 upon the supposition that light acts by vibra* 

 tions upon the retina, in the same manner as 

 the undulations of the air striking upon the 

 tympanum excite the sensation of sound. 



The velocity of the luminous undulations 

 deduced by Romer from the eclipses of the 

 satellites of Jupiter, is proved to be about 

 192,500 miles in a second; in other words, 



