NOVEJIBEE 8, 1895.] 



SCIENCE. 



629 



and mode of occurrence, and also in their cal- 

 careo-siliceous varieties, to tliose of Barbados." 



This previous publication clearly entitles Mr. 

 Gregory to the original anuouncemeut of the 

 radiolarian earths in Cuba, and had I known of 

 its existence lie should have received credit 

 therefor in my papers, and my work has only 

 resulted in the approximate determination of 

 their age and mode of occurrence. The nature 

 of Mr. Gregory's discovery is more fully ex- 

 plained, however, in a recent paper* in which 

 he clearly sets forth the fact that he found these 

 rocks, not in Cuba, but in Boston, in the collec- 

 tion made by Prof W. C. Crosby in the Museum 

 of Natural History, and refers to my prelim- 

 inary paper for the facts concerning their occur- 

 rence and age. In this paper he also presents 

 an interesting paleontologic study of this ma- 

 terial showing the presence of 17 families, 25 

 genera and 33 species. 



These are the facts concerning the knowledge 

 of the radiolarian beds of Cuba : The material 

 was first collected by Professor Crosby ; their 

 radiolarian nature determined from Professor 

 Crosby's collections in Boston by Mr. Gregory ; 

 their geological occurrence and age described 

 by the writer from studies on the ground, and 

 their specific paleontology determined by Mr. 

 Gregory. 



On page 311 of Vol. 51 of the Quarterly 

 Journal of the Geological Society of London, 

 dated August, 1895, as a portion of the discus- 

 sion following the delivery of Mr. J. "W. Gre- 

 gory's article on the Paleontology and Physical 

 Geology of the West Indies, Mr. A. J. Jukes- 

 Browne is quoted as follows : 



' ' In view of these facts, we are quite pre- 

 pared to accept Mr. Gregory's conclusion that 

 the Oceanic Series is of Miocene age, the more( 

 so as Mr. Spencer has come to the same conclu- 

 sion with respect to the Eadiolarian Earths of 

 Cuba, after a personal study of the geology of 

 that island." 



Inasmuch as Mr. Spencer makes no claim of 

 having ever visited eastern Cuba, and has only 

 seen the material from Baracoa which the 

 writer collected, the above paragraph is liable 

 to convey an erroneous impression concerning 



* Paleontology and Physical Geology of the West 

 Indies. Quarterly Journal of the Geological Society 

 of London, August, 1893, pp. 293-95. 



the discovery of the age and existence of these 

 important beds. Robt. T. Hill. 



Washington, D. C, October 16, 1895. 



ERECT VISION AND SINGLE VISION. 



Peop. Cattell's criticism of Prof. Brooks 

 (Science for October 11, p. 487) in the matter 

 of the inverted retinal image is undoubtedly 

 just, but his reply has not made things any 

 clearer. There is nothing specially inconceiv- 

 able nor specially inexplicable about erect vision 

 with inverted retinal image. It can be ex- 

 plained, too, without ' standing the soul on its 

 head.' This may be a metaphysical but surely 

 not a scientific explanation. In science what 

 we mean by an explanation is a reducing of the 

 phenomena in question to a law, which includes 

 many other phenomena and especially the most 

 common and familiar phenomena. Now, the 

 seeing things in their natural or real position by 

 means of inverted retinal images is a necessary 

 result of the law of direction, and this law is 

 the most familiar fact of common sensation. 

 Let me explain : 



Suppose I was standing on the plains of Ari- 

 zona, captive, bound and blindfolded, sur- 

 rounded by Apaches and a target for their ar- 

 rows. I think I could tell with reasonable cer- 

 tainty the general direction of the Apache who 

 shot any particular arrow. I would know it 

 by the part struck and especially by the direc- 

 tion of the push of the arrow. I would refer 

 the cause back along the line of the push to the 

 proper place. There is nothing especially in- 

 conceivable in this. Now, suppose I look at the 

 horizon. A star (I take this because it is a 

 point) sends its ray into my eye through the 

 optic center or nodal point, and it strikes, a cer- 

 tain rod or cone on the lower half of the retinal 

 concave. Is it anything specially strange that 

 the impression — the punch — should be referred 

 back along the line of the punch (ray line) to its 

 proper place in space and, therefore, that I 

 should see the star above the horizon? Now, 

 objects are made up entirely of such stars — i. e., 

 radiant points — each sending its ray straight to 

 the retinal concave, all crossing one another at 

 the nodal point and therefore making an in- 

 verted image. But each focal impression (focal 

 point) of the image is referred back along its own 



