630 



SCIENCE. 



[N. S. Vol. II. No. 45. 



ray line to its own place ; and thus the external 

 image is reinverted in the act of external reference, 

 and reconstructed in space in its true position as 

 the sign and facsimile of the object that made it. 

 These facts will perhaps be made still clearer 

 by the following diagram in which r, r, and sp, sp, 

 represent the retinal and spatial concaves. 



Every point, every rod and cone of the retina, 

 has its fixed correspondents in space, and these cor- 

 respondents exchange with one another by im- 

 pression and external reference. The arrow 

 and its retinal image are introduced to render 

 the subject, if possible, still clearer. Although, 

 indeed, I thought I had already made it suffi- 

 ciently clear in my littlebook 'Sight,' pp. 83- 

 89. 



It is seen, then, that there are two funda- 

 mental laws underlying monocular vision: 

 First, the law of external projection of retinal 

 impressions into space. Second, the law of di- 

 rection, i. e., the mathematically definite direc- 

 tion of this projection. These two laws explain 

 every phenomenon of monocular vision except 

 color perception. 



So again the apparent anomaly in Binocular 

 vision of single vision with two retinal images 



is, it seems to me, easily explained so far as 

 sense perception is explicable by science at all. 

 Those who observe closely their visual phe- 

 nomena know perfectly that except under well 

 known conditions we do see two objects or rather 

 two external images of every object, and we know 

 perfectly well which corresponds to, or is pro- 

 duced by, each retinal image. We see objects 

 single only luhen these two external images of the 

 same objects are placed one on the other and made 

 to coincide. This takes place when the two 

 retinal images of the same object fall on what 

 are called corresponding points of the two ret- 

 inae ; because then, by the law of direction already 

 explained, they are thrown to the same place in 

 space and their external corresponding images 

 coincide. Anyone accustomed to binocular ex- 

 periments can at will separate these images and 

 then bring them closer and closer, observing 

 them the while, until they coalesce and the ob- 

 ject is seen single. Is not this a sufiicient ex- 

 planation ? Joseph Le Conte. 

 Berkeley, Cal., October 17, 1895. 



SCIENTIFIC LHEBATUEE. 

 A Text-book of Mechanics and Hydrostatics. By 



Herbert Hancock, M. A., F. R. A. S., F. R. 



Met. Soc. New York, D. Van Nostrand 



Co. 1894. Pp. viii+409. 



It goes without saying that the task of pre- 

 paring a good elementary book on mechanics 

 is now far easier than at any jjrevious epoch in 

 the history of the science. The clarification in 

 fundamental ideas and the fixation of termi- 

 nology which have come about during the past 

 thirty years would seem to make it difficult for 

 an author to depart very widely ft'om sound 

 definitions and logical development. It is some- 

 what surprising, therefore, to find a book whose 

 author acknowledges his indebtedness to Max- 

 well, Thomson and Tait, and Clifford, marred 

 by the very confusion of ideas which those 

 eminent teachers have done so much to banish 

 from mechanics. 



Our suspicion of the author's fitness for his 

 work is raised in the first paragraph of his 

 preface, wherein he gravely affirms that "past 

 experience leads me to conclude that no com- 

 plete knowledge of mechanics can be got with- 

 out some knowledge (however elementary) of 



