CLARK: DISTRIBUTION OF ECHINODERMS 623 



and 



n = AUy = 

 1.57 X 1000 



(0.306 X 0.778 - 0.213 X 0.222) = 0.00049. 



0.61 X 10*^ 



That is, a thrust of looo kg. per sq. cm. changes the refractive 

 index 0.00049 for a ray vibrating perpendicular to the axis of 

 pressure and only 0.00020 for a ray vibrating in the direction 

 of the thrust. 



SUMMARY 



Glass when stressed becomes doubly refracting as shown by 

 its behavior in polarized light. This principle has often been 

 made use of in the detection and measurement of strain in glass, 

 but little has been known of the quantitative relation between 

 stress and birefringence. In this paper we have presented the 

 results obtained by loading blocks of glass in a testing machine 

 and measuring the concomitant birefringence. For all of the 

 glasses which we studied except the heaviest flint, a thrust of 

 one kg. per sq. cm. produces a birefringence of 2 to 3 X 10 ~'^; 

 or, in other words, a path-difference of 2 to 3 n/x per cm. thick- 

 ness of glass. 



Our results show satisfactory agreement with those of Pockels 

 for the flint glasses. 



ZOOGEOGRAPHY. — Discontinuous distribution among the echi- 

 noderms. Austin H. Clark, U. S. National Museum. 



While discontinuous distribution among terrestrial and fresh- 

 water animals has received a considerable amount of attention, 

 the same phenomenon among marine types has not been so 

 widely noticed. 



The following apparently anomalous ranges are occupied by 

 the genera of brittle-stars, starfishes and urchins listed ; there are 

 no similar cases among the crinoids; and the holothurians, on 

 account of the relatively unsatisfactory state of our present 

 knowledge, have been omitted. 



1. Warmer parts of the eastern and western Atlantic; Mexico 

 to Chile: Narcissia, Arbacia. 



2. Both coasts of tropical America: Encope, Mellita. 



