April 3, 1896.] 



SCIENCE. 



517 



As already announced by Prof. Rowland, it 

 appears that the anode is as important in the 

 matter as the cathode. We have a number of 

 tubes which give results, but none better than 

 the one mentioned, while a tube just received, 

 of American manufacture, promises to equal the 

 imported ones. 



The success so far obtained with the arm and 

 chest encourages us to think that still thicker por- 

 tions of the human body may be studied advan- 

 tageously, and experiments will be immediately 

 undertaken in this direction. 



Dayton C. Miller. 



Case School of Applied Science, 



March 25, 1896. 



[The photographs referred to by Prof. Miller, 

 like all others of a similar character, are diffi- 

 cult of adequate reproduction by photogravure. 

 The bones of the wrist and the large bones of 

 the forearm are splendidly shown and the 

 aluminum medal shows detail nearly as well as 

 an ordinary direct photograph. T. C. M.] 



THE INVERTED IMAGE ON THE KETINA. 



I CANNOT justly take to myself the severe re- 

 marks which Prof. Brooks makes, in the last 

 number of Science, concerning those who have 

 understood him to mean that there is something 

 peculiarly inconceivable in the inversion of the 

 image on the retina ; I did not myself take this 

 view, because I happened to know, before 

 writing my letter, that he disavowed this inter- 

 pretation of his words. I even fail to under- 

 stand by what rule of logic he drew the conclu- 

 sion that he was the distinguished scientist to 

 whom I alluded when I used these words : 

 ' ' Prof. Brooks can hardly hope that there 

 should be any consensus among scientific men 

 in regard to j(. ^{. ^ ^ ^. consciousness, if there are 

 still distinguished scientists who think that there 

 is anything which needs explanation in the fact 

 that the image on the retina is inverted." (I 

 add the italics now.) This view of the matter 

 is not uncommon, as the following instances, 

 in addition to the discussion which has been 

 going on for more than six months in Science, 

 and which Prof. Brooks has found so wearisome, 

 will indicate. A physician who had been trav- 

 ■elling among the Esquimaux recently reported 



before a medical society in Philadelphia that 

 those people are in the habit of holding a pic- 

 ture upside down when it is given them to look 

 at ; he accounted for this curious fact by sup- 

 posing that they were in such a low state of de- 

 velopment that they had not yet learned to re- 

 invert the image on the retina, and this hy- 

 pothesis was seriously discussed by this body of 

 physicians, without having its absurdity pointed 

 out by a single member. As another instance, 

 I mention that a prominent Baltimore physician, 

 in writing on the sensations of infants, lately 

 said that they see everything upside down at 

 first, and only learn afterwards to correct this 

 impression. 



Since Prof. Brooks has included me among 

 those who have failed to take his meaning as 

 he intended it, he cannot complain if I come to 

 their defence in a single word. He had said: 

 ' ' We all believe many things that are incon- 

 ceivable, such as the truth that the image in 

 the retina is upside down;" and again, "I 

 illustrated, by the inversion of the retinal image, 

 the fact that evidence may furnish conclusive 

 proof of truths that are inconceivable. ' ' Now, 

 while it is true that ' ' if, for purposes of il- 

 lustration, I declare my conviction that the 

 moon is not made of green cheese," no one has 

 a right to infer that I think the moon is made 

 of cheese of any kind, this supposititious asser- 

 tion offers no analogy to the case in hand. If 

 a person said that he could not believe that the 

 cheese of which the moon is made is green, and 

 also that he was not able to believe in the 

 greenness of the cheese of which the moon is made, 

 he would be using expressions precisely analo- 

 gous to those made use of by Prof. Brooks in 

 the case of the retinal image. Would anyone 

 be expected to use language like this, unless it 

 was the greenness only that troubled him ? 



C. L. F. 



necessary and SUFFICIENT TESTS OF TRUTH. 



Editor of Science : When Prof Brooks says 

 that it is a ' great law of logic that the test of 

 truth is evidence and not conceivability,' he 

 uses the phrase ' test of truth ' in a loose way 

 which (while it is not uncommon), in the inter- 

 ests of logic, I must protest against. 



To the mathematician it has long been a 



