November 24, 1916] 



SCIENCE 



749 



LATERAL VISION AND ORIENTATION 



To the Editor op Science : Professor C. C. 

 Trowbridge furnished an illuminating paper, 

 printed in Science September 29, on " The 

 Importance of Lateral Vision in its Eelation 

 to Orientation." 



In dealing with the question of the process 

 used by man, with his binocular frontal vision, 

 in estimating distances to objects that come 

 within his observation, Professor Trowbridge 

 says: 



It is a well-established principle that binocular 

 vision gives to human beings a means of determin- 

 ing the relative distances between near-by objects, 

 as well as the distances of these objects from the 

 observer. The basis of this power lies in seeing 

 the objects from two points of view, giving a 

 stereoscopic effect, which, however, is decreasingly 

 effective as the objects are removed from the eyes. 

 It is apparently partly the decreasing stereoscopic 

 effect with increasing distance which forms the 

 basis of measurement, and partly a judgment of 

 distance in some way through the muscular move- 

 ments of the eyes, and those governing the ac- 

 commodation of the lenses. . . . 



From the above quotation it appears to the 

 writer that Professor Trowbridge has missed 

 the fundamental principle of estimating dis- 

 tances to observed objects by human binocular 

 vision. If the writer's view or theory is cor- 

 rect, when a man estimates such distances by 

 his vision, he unconsciously performs a trig- 

 onometrical operation, in which the distance 

 between the pupils of the eyes is the base of a 

 triangle, the two lines of vision from the 

 pupils, converging in the observed object, being 

 the other sides of the triangle. 



The same principle is used by the " range- 

 finder " on a ship of war, who has a rod about 

 ten feet long as the base of his triangle, from 

 each end of which is measured the angular 

 inclination of the two lines converging in the 

 target, five or ten miles distant. The " binoc- 

 ular" observer has a base two and a half or 

 three inches long, for objects a few hundred 

 feet distant and less. The range-finder makes 

 accurate calculations based on measurements; 

 while the " binocular " observer, from long 

 practise, acquires a sort of " rule of thumb " 



facility in making such estimates with more 

 or less approximate accuracy, which operation 

 from long habit is performed intuitively and 

 without conscious mental effort. 



A man with only one eye, or with defective 

 vision in one of his eyes, finds a difficulty in 

 estimating the correct distance to an object 

 which he extends his hand to grasp; or when 

 inserting a key in a keyhole he must some- 

 times aid his vision by the touch of a finger to 

 locate the keyhole. 



It follows of course that a man with only 

 one eye is without the power to invoke the 

 principle of trigonometry in the estimating of 

 distances to observed .objects. 



If the above theory is unsound the writer 

 will be glad to have further enlightenment on 

 the question discussed. rp p -p» _„__ 



A COMMON, BUT INCORRECT, STATEMENT 



CONCERNING THE NUMBER OF 



BACTERIA IN MILK 



The literature discussing sanitary milk 

 problems is full of statements like this : " Cer- 

 tified milk is not allowed to have more than 

 10,000 bacteria per ex."; or " Grade A milk 

 should not have over 60,000 bacteria per c.c." ; 

 and many other similar statements specifying 

 the number of bacteria per c.c. in milk of vari- 

 ous grades. These counts are commonly made 

 by the standard agar plate method recom- 

 mended by the American Public Health Asso- 

 ciation. 



A perusal of a number of bacteriological 

 text-books by American authors shows a gen- 

 eral recognition of the fact that these counts 

 are probably counts of groups of bacteria 

 rather than of individual bacteria and that 

 they are probably always lower than they 

 should be because of the fact that not all bac- 

 teria will grow on nutrient agar at the incu- 

 bation temperature used. In spite of these 

 qualifications specifically stated in the majority 

 of these text-books, their authors ignore them 

 in all subsequent discussions and accept agar 

 plate counts as showing the number of bac- 

 teria per c.c. Occasionally in these books or 

 elsewhere in bacteriological literature, one 

 even finds the bald assertion that each colony 



