Maech 11, 1910] 



SCIENCE 



395 



glass face whereby a half wave-length is lost 

 at the mirror but not at the glass face in con- 

 tact, the fringes are impaired, making a 

 rather interesting experiment. With homo- 

 geneous light the fringes of the film itself ap- 

 pear to the naked eye, as they are usually very 

 large by comparison. 



Granting that the fringes in question de- 

 pend upon the reflecting surface behind the 

 grating, they must move if the distance be- 

 tween them is varied. Consequently a phe- 

 nomenon so easily produced and controlled is 

 of much greater interest in relation to micro- 

 metric measurements than at first appears and 

 we have for this reason given it detailed 

 treatment. It has the great advantage of not 

 needing monochromatic light, of being ap- 

 plicable for any wave-length whatever and of 

 admitting of the measurement of small hori- 

 zontal angles. 



When the phenomenon as a whole is care- 

 fully studied it is found to be multiple in 

 character. In each order of spectrum there 

 are different groups of fringes of different 

 angular sizes and usually in very different 

 focal planes. Some of these are associated 

 with parallel light, others with divergent or 

 convergent light, so that a telescope is essen- 

 tial to bring out the successive groups in their 

 entirety. At any deviation the diffracted 

 light is necessarily monochromatic; but the 

 fringes need not and rarely do appear in focus 

 with the solar spectrum. If the slit of the 

 spectroscope is purposely slightly inclined to 

 the lines of the grating, certain of the fringes 

 may appear inclined in one way and others 

 in the opposite way, producing a cross pattern 

 like a pantograph. The reason for this ap- 

 pears in the equations. 



In any case the final evidence is given when 

 the reflecting face behind the grating is mov- 

 able parallel to it. The principal fringes of 

 the interferometer so obtained are subject to 

 the equation (air space e, wave-length \, angle 

 of incidence i, of diffraction 6'), 



5e = X/2(cos 9' — cos i), 



and it is therefore less unique as an absolute 

 instrument than Miehelson's classic apparatus 



or the device of Fabry and Perot. Its sensi- 

 tiveness per fringe depends essentially upon 

 the angle of incidence and diffraction and it 

 admits of but 1 cm. (about) of air space be- 

 tween grating face and mirror before the 

 fringes become too fine to be available. But 

 on the other hand, it does not require mono- 

 chromatic light (a Welsbach burner suffices), 

 it does not require optical plate glass, it is 

 sufficient to use but a square centimeter of 

 grating film, and it admits of very easy 

 manipulation, for painstaking adjustments as 

 to normality, etc., are superfluous. In fact, all 

 that is needed is to put the sodium lines in 

 the spectrum reflected from the grating and 

 from the mirror into coincidence both hori- 

 zontally and vertically with the usual three 

 adjustment screws on grating and mirror. 

 Naturally sunlight is here desirable. There- 

 upon the fringes wiU usually appear and may 

 be sharply adjusted on a second trial at once. 



When the air space is small, coarse and fine 

 fringes (fluted fringes) are simultaneously in 

 focus, one of which may be used as a coarse 

 adjustment on the other. Finally the sensi- 

 tiveness per fringe to be obtained is easily a 

 length of one haM wave-length in the fine 

 fringes and one wave-length in the coarse 

 fringes, though the latter may also be in- 

 creased almost to the limit of the former. 

 C. Baeus, 

 M. Barus 



Brown UNrvEBsixT, 

 Providence, R. I. 



THE EFFECT OF ASPHYXIA ON THE PUPIL^ 



In a recent communication to the Society 

 for Experimental Biology and Medicine (p. 

 49, December 16, 1908) Dr. John Auer stated 

 that the " Myotic effect of asphjrxia in frogs 

 is interesting, as asphyxia in mamcmals pro- 

 duces chiefly dilatation." We were surprised 

 at this statement, as we had a different impres- 

 sion from having observed the pupils of vari- 

 ous animals during asphyxia. As such ob- 

 servations are usually recorded we examined 

 our protocols, and finding our impression con- 



^ From the physiological laboratories of Wash- 

 ington and Pittsburgh universities. 



