December 31, 1886.] 



SCIEJS'CE, 



631 



British medical journal, in which, while blowing 

 out a match, the patient's 'breath caught fire with 

 a noise like the report of a pistol, which was loud 

 enough to awaken his wife. One evening, while 

 a confirmed dyspeptic was lighting his pipe, an 

 eructation of gas from his stomach occurred, and 

 the ignited gas burned his mustache and lips. 

 In Ewald's book oa indigestion, the analysis of 

 the gas in one of these cases was, carbonic acid, 

 20.57; hydrogen, 20.57; carburetted hydrogen, 

 10.75 ; oxygen, 6.72 ; nitrogen, 41.38 ; sulphuret- 

 ted hydrogen, a trace. The origin of these gases 

 is undoubtedly the undigested food, which in 

 these cases undergoes decomposition. 



— Dr. Gilles de la Tourette finds that the aver- 

 I age step of men is twenty-rive inches ; for w^omen, 

 twenty inches. The step with the right foot is 

 somewhat longer than that with the left. The 

 feet are separated laterally in walking about four 

 and one-half inches in men, and five in women. 



LETTERS TO THE EDITOR. 



*t* Correspondents are requested to be as brief as possible. The 

 writer's name is in all cases required as proof of good faith. 



Cremona's Projective geometry. 



YouE review of this work does scant justice, I think, 

 to"one of the most valuable text-books recently pub- 

 lished. We have a ixiultitude of elementary books in 

 all branches of science ; but why most of them are 

 printed, there seems to be no reason, unless it be the 

 reason why cheap razors are made. For my own 

 part, I am thankful when we get a book such as Pro- 

 fessor Cremona has given us, — a book so well de- 

 signed to give the student more general views of 

 geometry. Asaph Hall. 



Washington, D.C., Dec. 28. 



Pleuro-pneumonia. 



Referring to Mr. Butler's communication and your 

 editorial remarks on p. 587, it may be of interest to 

 put on record the fact that horses have suffered quite 

 extensively, particularly in Indiana and Missouri, 

 from what Dr. Salmon has decided to be vermicular 

 or verminous bronchitis. He has fully treated of this 

 disease, and illustrated the Strongyli which induce it 

 in calves and lambs, in the veterinary part of the 

 'Agricultural report for 1885.' That producing the 

 dispase in horses seems to be Strongylus micrurus 

 iiie;il:s, which is carefully figured on plate V., 

 and described on p. 557. It is an elongate, thread- 

 like worm from an inch and a half to two inches 

 in length; and the point that I wish to put 

 on record is that these Strongjdi have very 

 generally been supposed to have some connec- 

 tion with the narrow elongate eggs of Orchelimum 

 glaberrimum. The eggs of this si^ecies are inserted 

 in the pith of a number of different plants, and are 

 particularly abundant in stalks of corn-tassels. The 

 punctures were figured in my ' Fifth rejDort on the 

 insects of Missouri,' and again referred to in bulletin 



6, U. S. fish commission The bronchial disease 

 which has been so prevalent and fatal to horses has 

 been quite generally associated with these eggs, the 

 supposition being that the horses became diseased 

 by eating the corn tassels and stalks. The Orcheli- 

 minn eggs have been received from about a dozen 

 different correspondents, all of them independently 

 making the same suggestion as to their connection 

 with the bronchial worms, a rather remarkable in- 

 stance of a prevalent and popular error arising from 

 an imperfect knowledge of natural science. 



C. V. ElLEY. 



Washington, D.C., Dec. 27. 



Stereoscopic vision. 



I would like to inquire of the readers of Science if 

 it is generally known to be possible — and if, indeed, 

 it is possible to all persons — to obtain a complete 

 stereoscopic effect in viewing a single picture, and 

 without a glass or other instrumental aid. 



I have for several years been in the habit of prac- 

 tising a method in looking at photographs or good 

 engravings, which, with me, makes the illusion per- 

 fect, and the objects pictured seem to stand out in 

 full relief like the real objects. 



In consists simply in entirely closing one eye, and 

 shutting the other as nearly as possible, while admit- 

 ting just sufficient light to afford a distinct, or at first 

 rather dim, view of the picture. It is necessary first, 

 however, to see that the picture is jjlaced in a light 

 corresponding as accurately as possible in direction 

 with that in which the objects are represented in the 

 picture : for example, if the scene is shown as lighted 

 from the left, let the picture be so held that the actual 

 illumination is from the left, and exactly at the same 

 angle. An incongruity in this resjDect will spoil the re- 

 sult entirely. A little time is usually required to realize 

 the fiill effect, and probably many persons imaccus- 

 tomed to the experiment will need to exercise more 

 patience at first than after some practice. 



It is found, too, that a j^icture iDresenting strong 

 lights and shades, as of photographs of objects in 

 the direct sunlight, or engravings of the same char- 

 acter, produces the effect most readily. Take, for 

 example, the engravings representing highly magni- 

 fied views of the scenery on the surface of the moon, 

 such as those illustrating Professor Langley's article 

 ' The new astronomy,' in the Century. After looking 

 at one of those in that manner for a few moments, 

 the iDarts represented as elevations appear to rise 

 from the pajDer; and, indeed, the flat surface disap- 

 pears altogether, as well as the inky blackness of 

 the shadows, and both elevations and dej^ressions 

 appear in startling reality. 



The lights and shadows appear to be merely the 

 illuminated and unillumiuated portions of the same 

 uniformly colored siibstance, showing it distinctly 

 carved in all the realitj^ of the forms intended to be in- 

 dicated. It seems as if one could closely estimate the 

 actual heights of the elevations, and the lengths of 

 the shadows, and the jirecise position of the source 

 of light. 



The illusion once perfected, it may be retained 

 while opening the eye a little, thus gaining a clearer 

 view ; but, carrying this a little too far, the scene at 

 once ' flattens out ' again, and becomes a mere lifeless 

 black-and-white representation of the outlines, pro- 

 ducing nothing of the impression of reality of con- 

 torn- : the landscape is gone. 



