August 2G, " 1904.] 



SCIENCE. 



279 



radiomicrometer, and his measurement of the 

 energy radiated from certain stars and planets, 

 are properly acknowledged. 



The chapter on optical instruments is ex- 

 cellent. It is followed by one on the eye, 

 which furnishes a satisfactory summary of 

 what is of chief interest in physiological 

 optics to the physicist. This includes a brief 

 discussion of the telestereoscope of Helmholtz, 

 and the more recent double telescope by Zeiss 

 for the perception of a distant object in relief. 

 Interesting developments are the stereo- 

 telemeter, stereo-micrometer and stereo-com- 

 parator of Pulfrich. This last instrument 

 finds a new and unexpected field of application 

 to the heavenly bodies. Assume that on two 

 successive evenings photographs of Saturn are 

 taken, using the comparator with camera at- 

 tachment. During the interval of a day the 

 position of the planet with regard to the stars 

 has changed, as well as the position of the 

 satellites with regard to their primary. Let 

 these two photographs be arranged to form a 

 stereograph and viewed binocularly either with 

 a stereoscope or with the unaided eyes if suit- 

 ably trained. Against the black background 

 are seen the distant stars. Suspended inde- 

 pendently in mid space between foreground 

 and background is the planet. Behind it on 

 one side is a satellite, and on the other side is 

 another satellite just emerging from eclipse. 

 Spatial relations are as distinct as if all were 

 within arm's length. A stereograph of this 

 kind from proofs secured by Wolf at the 

 Heidelberg observatory is one of several that 

 are presented for the reader's scrutiny. The 

 instrument has been applied to the discovery 

 of planetoids, of variable fixed stars, and to 

 the study of such as have considerable proper 

 motion. 



The last part of the volume, relating to 

 optical phenomena in the atmosphere, inter- 

 ference of light, diffraction, polarization, 

 double refraction, interference of polarized 

 beams and the turning of the plane of polar- 

 ization by quartz and other bodies optically 

 active, is well up to the standard of the earlier 

 part. The value of the book is greatly en- 

 hanced by the clear cut summaries of impor- 

 tant conclusions, and the bibliographic list 



of references to the literature of the subject 

 vyith which each chapter is closed. 



W. Le Conte Stevens. 



DISCUSSION AND CORRESPONDENCE. 



PALEOZOIC SEED PLANTS. 



In my short note in Science of July 1 pro- 

 posing the name Pteridospermaphyta for this 

 group of plants, I assumed that all interested 

 in the subject were acquainted with the facts 

 and the literature, and I expressly refrained 

 from entering into details. It seems that I 

 was mistaken in this assumption, otherwise I 

 could hardly have been misunderstood. 



When in 1897 Potonie founded the group 

 Cycadofilices,* he based it on the internal 

 structure and classed it under the Pteridophyta 

 with the same ranis as the Filices. It included 

 Nosggerathia, the MedullosaB, Cladoxylon, 

 Lyginopteris, Heterangium and Protopitys. 

 Later he worked the same subject over for 

 Engler and Prantl's ' Natiirliche Pflanzen- 

 familien.'t He here says that the groups 

 Sphenopterides, Percopterides and ISTeuropter- 

 ides might perhaps be better included in the 

 Cycadofilices, although he continues to class 

 them with the ferns. He now includes Calam- 

 opitys in this group. M. R. Zeiller in 1900:(: 

 discussed these forms, and although he ad- 

 mitted that the characters then known ap- 

 proached more closely those of cycads than of 

 ferns, he says it would be rash to exclude them 

 from the latter on these characters alone, that 

 they may represent a special type of Filicinese, 

 provided with secondary wood, and that indi- 

 cations of fructification observed on certain 

 fronds of Alethopteris, Odoniopteris and iVeur- 

 opteris may be adduced in favor of this 

 hypothesis. 



In the first preliminary paper of Drs. Oliver 

 and Scott§ they show that Lyginodendron, 

 which Potonie classes in the Lepidodendracese, 



* ' Lehrbueh der Pflanzenpalaeontologie,' p. 160 

 (Lief, 2, dated 1897). 



t Teil I., Abt. 4, pp. 780-795 (Lief. 213 dated 

 1902). 



t ■' Elements de Paleobotanique,' pp. 124 ff., 370. 



§ ' On Lagenostoma Lomaxi, the seed of Lygino- 

 dendron,' Proc. Roy. Soc, Vol. LXXI., pp. 477- 

 481. 



