14 



8CIUNCE, 



[Vol. IX., No. 205 



liimself at station No. 12, on one of the peaks of the 

 San Juan Mountains, in August, 1874, at an altitude 

 of 13,967 feet above the level of the sea. 



An interesting and significant circumstance recorded 

 by Mr. Ehoda was the fact that there was a sudden 

 and instantaneous cessation of the distressing electri- 

 cal manifestations whenever a stroke of lightning 

 took place, to be speedily renewed by the returning 

 tension of the electricity. He says, " The sharp 

 points of the hundred stones about us each emitted 

 a continuous sound, while the instrument outsang 

 every thing else, and, even at this high elevation, 

 could be heard distinctly at the distance of fifty yards. 

 The points of the angular stones being of different 

 degrees of sharpness, each produced a sound peculiar 

 to itself. The general effect of all was as if a heavy 

 breeze were blowing across the mountain. The air 

 was quite still, so that the wind could have played no 

 part in this strange natural concert, nor was the in- 

 tervention of a mythological Orpheus necessary to 

 give to these trachytic stones a voice." 



John LeConte. 

 Berkeley, Cal., De'c. 25. 



Stereoscopic vision. 



In reply to the inquiry of Mr. W. H. Pratt in the 

 last issue of Science, it is necessary only to consider 

 the various elements which are combined in the 

 formation of a visual judgment. If an observer, 

 who possesses but a single eye, looks out upon a 

 landscape, the relative distance of the different ob- 

 jects viewed may be roughly estimated in terms of 

 some standard arbitrarily chosen, so long as they are 

 not precisely aligned with his eye. The judgment 

 is less accurate as the angular separation of the ob- 

 jects becomes less, and as there are fewer of them at 

 moderate distances for comparison with the rest. 

 Always, and usually unconsciously, he employs one 

 or more of the following elements in judging the 

 distance and form of each object regarded : — 



I. Near objects subtend larger visual angles than 

 remote objects of equal size. 



II. Near objects are seen more distinctly than 

 those that are remote. The illusion of distance 

 may hence be produced by decreasing the brightness 

 of the object viewed, by changing the nature of the 

 medium, or by increasing the contrast between light 

 and shade. 



III. Near objects that are almost aligned with 

 those which are remote, often partly cover them. 

 Covering objects are judged nearer than those 

 covered. 



IV. Familiarity with the dimensions of known ob- 

 jects when near enables us to compare them when 

 remote, and thereby judge their relative distance. 



V. By moving from one stand-point to another, 

 and comparing the new view with what is retained 

 in memory of the previous one, parallax of motion 

 thus contributes to the formation of a judgment of 

 both distance and form. 



All of these elements may be imitated in pictures, 

 except the last. In the examination of ordinary stereo- 

 graphs they are combined vdth the important element 

 of binocular perspective, and to such an extent that it 

 is impossible to know just how much we are indebted 

 to binocular perspective for the illusion of apparent 

 relief. Skeleton diagrams, properly constructed, 

 are hence the only means of studying stereoscopic 



vision, if this term be taken as a synonyme of binoc- 

 ular vision. If Mr. Pratt will try his method with an 

 outline drawing, it will fail. 



In regarding an ordinary painting, binocular vision 

 is often a hinderance, rather than an aid, in appreci- 

 ating perspective. It is at least important to cut off 

 from view the objects surrounding the picture, which 

 we involuntarily take into comparison with it. In 

 the application of geometry to perspective, a single 

 point of view (station-point) is always assumed, and 

 in examining the result the observer should place a 

 single eye as nearly as possible at the same station- 

 point to attain the best perspective illusion. The 

 other eye must be closed, if he wishes to exclude 

 the interfering element of binocular vision which 

 will at once be unconsciously apialied to the card or 

 canvas on which the picture has been made. 



It is by the observance of these precautions that 

 Mr. Pratt has been able to appreciate perspective in 

 the pictures examined, but true stereoscopic vision 

 was excluded instead of being attained by what he 

 may have supposed to be a new method. 



W. LeConte Stevens. 

 Brooklyn, Jan. 1, 1887, 



Star rays. 



Mr. Randolph will find the phenomenon of the 

 long vertical rays or streamers proceeding from a 

 strongly luminous point described and fully ex- 

 plained in my little volume entitled ' Sight,' pp. 87- 

 89. They are produced, not by reflection from the 

 eyelashes, as he supposes, but by refraction of light 

 passing through the meniscus of moisture between 

 the lid and the cornea, and are therefore more dis- 

 tinct when the lids are brought near together, I had 

 investigated the phenomenon and ascertained its 

 cause before I was aware of the very brief mention 

 of it in Daguin's ' Traite de physique,' vol. iv. p. 323. 



The radiating points about a star are more difficult 

 to explain. They are probably due to some pecul- 

 iarity in the structure of the crystalline lens. 



Joseph LeConte. 

 Berkeley, Cal., Dec. 35. 



A German sentence. 



In your current number you give an example of a 

 German sentence. In Teutonicity it can hardly com- 

 pete with the following extract from an advertise- 

 ment of a well-known periodical : ' ' Als eines der 

 vorziiglichsten Weihnachtsgeschenke miissen die ele- 

 gant gebundenen Quartalsbande der Deutschen 

 Eundschau herausgegeben von Julius Eodenberg 

 Preis pro Band in elegantem, rothem Originallein- 

 wandband mit Schwarz und Golddruck 8 Mark be- 

 zeichnet werden." N. 



Washington, Jan. 3, 1887. 



Pleuro-pneumonia. 



It may hot be worth while to call attention to two 

 slight mistakes in the printing of my communication 

 on p. 631 (viii. No. 204). The 'meplis' should be 

 ' Mehlis,' the author of micrurus; and the 'U. S. fish 

 commission ' on the first line of second column should 

 be ' U. S. entomological commission.' 



C. V. Riley. 

 Washington, D.C., Jan. 3, 1887. 



