OCTOBEE 11, 1895.] 



SCIENCE. 



487 



(p. 15, 16). Yet, even Maxwell speaks of 'heat 

 rays,' almost as if they objectively possessed heat, 

 and, of course, with the implication that ' rays 

 of light ' are not ' heat rays. ' Can it be urged 

 too strongly that the rays diifer objectively only 

 in wave-length and amplitude, and that their 

 relations to heat and light are entirely and ab- 

 solutely subjective? Yet loose jihrasing is con- 

 tinually met with. Langley writes of 'lumin- 

 ous heat ' and of ' the radically different char- 

 acter of the heat in these two maxima ' (A. J. 

 S., ut supra, 434, 435). Hallock writes : "Then 

 it was that heat was recognized as another 

 manifestation of those periodic disturbances, or 

 waves, in that elastic medium which was then 

 known as the luminiferous ether, and which is 

 now universally linown as ' the ether ' (Sci- 

 ence, ut supra, 174). Perhaps this refers to 

 the mis-recognition of the early part of this cen- 

 tury ; perhaps professional physicists get along 

 comfortably enough with ' dark heat rays ' and 

 the rest ; but to those who have to use physical 

 results in other lines of study, this indefinite 

 phraseology is very troublesome. 



W. M. Davis. 

 Harvard University, Sept. 30, 1895. 



SHELLS AS IMPLEMENTS. 



Please call the attention of those who own 

 or have charge of archaeological cabinets to an 

 illustration in von den Steineu's ' Unter den Na- 

 turvolkern Zentral Brasiliens,' 1894, p. 207, 

 fig. 27. A fresh water mussel shell has a hole 

 through it just as you see in specimens on plate 

 xxvi. of Holmes' paper. 'Art in Shell.' But 

 von den Steinen says that these shells are used 

 as scrapers ; the edge on large objects and the 

 hole through the shell is also used by the tribes 

 living on the upper Shingu for smoothing or 

 scraping wood. His next remark about push- 

 ing the hole in with an Akuri nut I do not 

 comprehend. O. T. Mason. 



THE INVERTED IMAGE ON THE RETINA. 



In the last number of this journal (p. 438) 

 Professor Brooks writes : ' ' We all believe 

 many things which are inconceivable, such as 

 the truth that the image in our eyes is upside 

 do-wn." But why is this inconceivable? To 



those having knowledge of elementary physics 

 it is inconceivable that the image should not be 

 inverted. Perhaps Professor Brooks means 

 that it is incomprehensible that we should see 

 things right side up when the image is upside 

 down. This is sometimes urged, but would 

 seem to be sufficiently answered by a remark 

 once made by Lotze in the presence of the 

 writer : "If any one is troubled by the fact 

 that the image is inverted, let him suppose that 

 the soul stands on its head." It is, indeed, 

 quite as reasonable to suppose that the mind 

 stands on its head as to suppose that it stands 

 on its feet and looks at the image on the retina 

 — which would seem to be the assumption of 

 those who are troubled by the phenomenon. 



A similar paradox is the fact that with two 

 images on the retinas we see things singly. This 

 may also be treated without undue seriousness 

 by the question : "If we hear a baby crying 

 with two ears, why do we not think it is twins ? 

 J. McK. C. 



SCIENTIFIC LITERATURE. 



Mental and Physical Fatigue by M. Mosso. 

 Translation by P. Langlois. (Bibliotheque 

 de philosophie contemporaine.) Paris, Felix 

 Alcan. 1894. 

 The Difference Between the Muscles in Their 

 Normal and Their Abnormal, or Fatigued Con- 

 dition by M. Wedensky. Archives de physi- 

 ologic ; Comptes rendus de 1' Academic des 

 Sciences. 



It is but recently that problems of this nature 

 have been treated by physiologists. 



Kronecker, in his experiments on the detached 

 muscles of the frog, succeeded in obtaining 

 1,000, even 1,500 contractions, the intensity of 

 which decreased regularly in proportion to the 

 increase of fatigue ; thus, for contractions at 

 regular intervals, produced by currents of equal 

 intensity, the curve of fatigue is a straight line. 

 Kronecker also observed the great individual 

 difierences existing in animals in their power 

 of resistance to fatigue. 



M. Mosso, the author of the present work 

 (unfortunately abridged in the French transla- 

 tion), is an Italian physiologist who has under- 

 taken with an instrument of his own invention, 



