December 10, 1915] 



SCIENCE 



841 



Giardia muris and Giardia microti produce 

 a readily recognizable enteritis in mice, and 

 both binary and multiple fission take place in 

 tbe free non-encysted stage — there is no Oc- 

 tomitus stage. The morphological characters 

 separate six species. The parasite in mice ap- 

 pears to be distinct from that in man. 

 4. The Inorganic Constituents of Alcyonaria: 



F. W. Clarke and W. C. Wheeler, United 



States Geological Survey, Washington. 



The stony corals have been repeatedly 

 analyzed, and with generally concordant re- 

 sults. Thirty analyses here made have con- 

 firmed the older data. The object of the in- 

 vestigation is to determine what each group 

 of organisms contributes to the formation of 

 marine limestones. The highest proportions 

 of ealeitim phosphate are commonly associated 

 with high values for magnesia. 

 6. An Experimental Analysis of the Origin 



and Relationship of Blood Corpuscles and 



the Lining Cells of Vessels: Charles E. 



Stookard, Department of Anatomy, Cornell 



University Medical School. 



Vascular endotheliiun, erythrocytes and leu- 

 cocytes, although all arise from mesenchyme, 

 are really polyphyletic in origin; that is, each 

 has a difierent mesenchymal anlage. 



Edwin BrowELL Wilson 



SPECIAL ABTICLLS 



interferences with two gratings 

 If two identical grating are placed with the 

 ruled faces and rulings in parallel and the 

 horizontal and transverse axes of their spectra 

 (of the same side and order) in coincidence, 

 white light passed through them from the 

 collimator of a spectrometer shows intense, 

 nearly equidistant, vertical interference 

 fringes in the telescope. The path difference 

 is subject to the equation e(l — cos 6), where 

 e is the distance apart of the ruled faces and 

 the angle of diffraction. These fringes there- 

 fore belong to the coarse set which I described 

 elsewhere. Though not exceptionally sensitive 

 to displacements of either grating, they are 

 available throughout a relatively large inter- 

 val; i. e., e may be increased from coincidence 



to over 2 em. As two stretched films suffice, 

 these strong fringes admit of many practical 

 applications. 



A more interesting class of fringes may be 

 observed, when the light used in the same in- 

 strument is homogeneous. There are three 

 types of these fringes of constant wave-length. 

 The first of these is obtained with the same 

 adjustment for coincident longitudinal and 

 transverse spectrum axes, but needs a wide slit. 

 Obliquity of the incident rays here replaces 

 the above color difference. The second class 

 appears with a fine slit, coincidence of longi- 

 tudinal axes, but in the absence of coincidence 

 of transverse axes (in which adjustment the 

 fringes would be of infinite size). They are 

 thus evoked by a difference in the angle of 

 incidence at the two gratings, respectively. 

 Frequently they are seen to best advantage 

 with the naked eye or a lens. They increase 

 in size as the eye is withdrawn from the grat- 

 ing; or if seen in the telescope, if the ocular 

 is either pulled out or pushed in from the 

 position for the principal focus where D lines 

 only are seen. For any given position of the 

 eye they do not vary in size while either 

 grating is displaced from coincident position, 

 to the position of vague evanescence, 4 or 5 

 millimeters beyond. Both this and the fol- 

 lowing fringe patterns rotate rapidly with the 

 slight rotation of either grating in its own 

 plane. 



The third class is obtained in the absence of 

 a collimator and is due to the varying obliquity 

 of diffuse homogeneous light. The longitu- 

 dinal spectrum axes must coincide, but the 

 transverse axes need not. They are very 

 strong, best seen with the naked eye or lens, 

 but admit of relatively little displacement of 

 either grating, as they vanish with increasing 

 smallness. They usually lie in a definite focal 

 plane, which recedes to infinity as the gratings 

 are more and more separated. 



Finally it is interesting to note that phe- 

 nomena of a somewhat similar kind may bo 

 obtained with reversed spectra. 



Gael Barus 

 Beown University, 

 Providence, K. I. 



