SCIENCE 



NEW YORK, JUNE 10, 1893. 



DIRECT REFLECTING POLARISCOPES. 



Polarization by reflection is more perfect than by trans- 

 mission through thin plates, unless a large number of plates 

 are used, and in that case there is difiiculty in finding plates 

 free from color. The disadvantages of reflection are (1) the 

 "elbow" angle and (2) the impossibility of rotation of the 

 polarized beam. Both these objections are overcome in 

 the forms here described, which may be attached to the lan- 

 tern by a sliding collar and rotated almost as easily as a 

 Nicol. 



In Fig. 1, p is a bundle of thin glass plates, set at the 

 polarizing angle; m is a silvered mirror. Either the reflected 



or the transmitted beam may be used; or, if the mirror is 

 slightly movable, the two images may be thrown either side 

 by side or superposed upon the screen. 



In Fig. 2 the bundle of plates, p, has a black backing, and 

 there are two silvered mirrors, m, m. The reflected beam 

 only is used. 



The form shown in Fig. 3 is more complicated and clumsy 

 in appearance, but it has the advantage of keeping either 

 the reflected or the transmitted beam, or both, in the axis of 

 rotation, a and 6 are movable blackened screens. 



T. Proctor Hall. 



Clark University, Worcester, Mass. 



Professor A. S. Hardy of Dartmouth, who has been spoken 

 of for president of the college, has decided to leave Hanover and 

 take a neve professorship at West Point. 



NOTES ON THE FERTILITY OF PHYSA HETERO- 

 STROPHA SAY.' 



On the 8th of March. 1886, I collected from a marsh near 

 Wake Forest two specimens of Physa heterostropha Say. 

 On the I6th three thick nidamenta, of some forty eggs each, 

 were seen loosely attached to the walls of the glass aquarium. 

 A few days later four others had been deposited. Up to June 

 15 the aquarium was examined at intervals nearly every day. 

 After that date it was not seen again until July 12, when the 

 water was changed. The next day both the snails were dead, 

 probably as the result of the change of water. 



In the period of four months — say March 12 to July 12 — 

 the pair produced 43 nidamenta, which contained, on an esti- 

 mate certainly not too high, an average of 30 eggs each, so 

 that the number of their offspring for the period mentioned 

 amounted to 1,290. There was no well-marked decline of 

 the reproductive function toward the close of the period, 

 which is perhaps another indication that they came to their 

 death by violence. 



From March 31 to June 6 inclusive, the pair were observecj 

 in coitu as many as fifteen times, at hours ranging fromj 

 8.30 A.M. to 6.15 P.M., the coitus lasting sometimes but twenty 

 minutes, sometimes more than an hour. The male function 

 •was performed alternately by the two snails. The eggs ap- 

 pear to have been laid only during the night. 



It was important to determine, if possible, the age at which 

 sexual maturity is attained and reproduction begins. Ac- 

 cordingly, on the 12th of July I took out of the aquariumi 

 two of the largest of the young snails and put them into an- 

 other aquarium. They were presumably members of the 

 first brood, the eggs of which were deposited near March 13. 

 Their age, reckoning from the time they were hatched, was 

 about 3|- months; size, length of shell, 5 millimetres; length 

 of foot, 6 millimetres. In two days one of the snails was 

 dead. On the 25th of July another snail of about the same 

 size was introduced from the first aquarium. The next entry 

 in my notes is under date of Sept. 11, when six nidamenta 

 were observed attached to the fibrous roots of a water plant. 

 They were, however, small, containing only from one to 

 four eggs each, showing that the reproductive function at 

 that age was feeble. Some of the eggs were already hatched, 

 and the tiny grandchildren of my first Physas were going 

 about the aquarium in search of food. Allowing, say, fif- 

 teen days for the intracapsular development of these snails 

 of the third generation, I estimate that the isolated pair of 

 the second generation attained sexual maturity at five months 

 of age. The same day — Sept. 11 — in the first aquarium I 

 noticed a confirmation of my observation in the second, 

 namely, the pairing of two of the oldest brood. 



The maintenance of a species depends on the equilibrium 

 between the forces tending to its destruction and those tend- 

 ing to its preservation. "We may enbrace the former under 

 the general phrase, adverse external conditions. There are 

 two difi^erent ways in which the destructive tendency of these 

 adverse external conditions is opposed. The first is by adap- 



' Abstract of a paper rood before the EUsha Mitchell Sclentlfle Society In 

 session at Wake Forest, . ., Oct. v3, lt>91. 



