February 3, 1888.] 



SCIENCE. 



61 



any two points in monocular vision. Images upon different sides 

 of the fovea in monocular perception never combine, and are never 

 supposed to combine. Now, supposing C and D in the fovea o 

 and c' by convergence, and keeping in mind the fact that the tem- 

 poral half of the right eye in binocular perception corresponds to 

 the nasal half of the left in monocular perception, the images of A 

 and B, while they fall in non-corresponding halves, occupy positions 

 visually the same as If they fell upon non-corresponding halves in 

 monocular perception, the temporal and nasal ; and hence, superim- 

 posing L upon j'?, e would fall as far from the fovea in the nasal 

 half as a from the fovea in the temporal half of the left eye L : that 

 is, the images of A and B, a and e , visually fall upon opposite 

 sides of the fovea, and can no more combine than separate images 

 in monocular perception. 



The same general result is obtained if we combine C and D by 

 diverging the eyes ; that is, by focusing the eyes in the median line 

 beyond the point E, or beyond the stereoscopic figures. The eyes 

 are thus turned outwards, so that the fovea in each case must be 

 shifted inward from c to d, and from c' to b' . Combination of C 

 and D will thus be attained by intra-foveal images, — such as are 

 intra-foveal while the point of fixation remained at E. But when 

 (f and b' are brought into their corresponding fovea, e and a' still 

 remain intra-foveal at distances from the fovea equal to that be- 

 tween d and e, and a and b' . By the same argumentation as be- 

 fore, it can be shown that the images of A and B, respectively e 

 and a ', cannot combine. Thus, being both intra-foveal, they fall 

 upon points in the nasal halves of the two eyes. These are binocu- 

 larly non-corresponding, and therefore monocularly complementary 

 halves of the retina : hence falling upon e and a ' in binocular vision 

 is the same as occupying opposite sides of the fovea in monocular 

 vision, and so combination will be impossible. This shows the im- 

 portance of observing what is implied by the term ' disparate.' As 

 long as we conceive the term in its binocular application, there 

 would be some reason for supposing combination upon them under 

 the circumstances described. But adjustment by convergence and 

 divergence, the former for extra-foveal and the latter for intra-foveal 

 images, requires us to think of ' disparate ' in its monocular applica- 

 tion ; and in that case we must either deny the possibility of combi- 

 nation upon them, or abandon the whole theory which makes a 

 nasal half of one eye correspond to a temporal half of the other ; 

 for, if ' disparate ' points in monocular perception may admit of 

 combination, a nasal half may correspond to a nasal half, and a 

 temporal to a temporal half, of the retina. This has never been 

 assumed to be possible. 



Of course, ' intra-foveal ' and ' extra-foveal ' are used with refer- 

 ence to the vertical meridian, and not the horizontal meridian, as 

 Fig. 4 would seem to imply. In the last figure A and B represent 

 • positions relative to the vertical meridian of any objects in the tem- 

 poral halves of the retina, and hence they may be above or below 

 the horizontal meridian in which they really lie, according to the in- 

 cHnation of the lines to the median plane. The modification for 

 the nasal halves of the retina can be supplied by the reader. It is 

 evident from this that this demonstration does not apply mathe- 

 matically to Fig. I, where the apparent fusion is of binocularly 

 ' disparate ' points, although, taken in the total sense for localiza- 

 tion, it will apply. But it is combination, not localization, that we 

 are discussing. 



If the stereoscope is used to effect the combination, the perspec- 

 tive noticed in convergence with the naked eyes is reversed, and is 

 identical with that effected by the divergent movement to produce 

 combination. The reason for this may be briefly stated. The 

 partition between the lenses lies in the median line, and hence cuts 

 off the extra-foveal images entirely. Combination has therefore to 

 be effected by the intra-foveal. With this statement of the condi- 

 tions, the argument could be carried out as before. 



But the reply to our position that stereoscopic combination upon 

 ' disparate ' points must be impossible, will be the very plain one 

 that it contradicts the /«<:/.? of actual vision; that we can actually 

 see the combination to have taken place ; and, since it cannot have 

 been upon corresponding, it must have been upon ' disparate ' 

 points. There are two replies to this, and, in addition, an impor- 

 tant fact which explains the apparent anomaly. In the first place, 

 the demonstration is mainly intended to show that the phenomenon 



must be impossible if we still retain the ordinary theory in regard 

 to the divisions of the retina and their functions. In the second 

 place, experiment shows that our claim is correct : for, after long 

 practice in combination by convergence or by divergence, those 

 images which, according to construction, must fall upon disparate 

 points, and which at first seemed to be single and to coincide, ap- 

 pear double until they are brought into the fovea. This indicates 

 that they were never really fused into one. Why, then, is the fusion 

 so apparent to vision ? The answer is, that inhibition had sup- 

 pressed such portions of one or both images contending for fusion, 

 that the resultant, made up of complementary elements, appears as 

 a single image. After considerable practice, the reflex and auto- 

 matic tendency is weakened, and inhibition correspondingly de- 

 creases ; so that the images which before seemed single appear 

 double, as the law of disparate points requires. 



Baltimore, Md., Jan. 4. J. H. HYSLOP. 



Bacteriology as a Study in Schools. 



The subject of the study of bacteria, discussed by Professor 

 Conn in a recent number of Science (xi. No. 257), is one which de- 

 serves more attention than it has attracted thus far, and I take the 

 liberty of making a few suggestions which have presented them- 

 selves to an investigator rather than a teacher, but which may 

 prove useful to the latter. Let us call the subject ' bacteriology ' for 

 convenience' sake, an4 drop the misleading expression ' germ- 

 theory of disease,' which has had its day. We know, as positively as 

 we know that the earth revolves on its axis, that certain diseases in 

 man and animals are caused by the invasion and multiplication of 

 bacterial parasites. There is no theory about this. The phrase is 

 misleading, because it states that all disease may be due to germs, 

 which is manifestly untrue. 



There are several classes of students who would be greatly bene- 

 fited by a careful study of bacteria in the laboratory. 



1. Students of general biology and physiology would gain by 

 a few simple experiments, readily performed, a very clear insight 

 into the great metabolic activity of life in general, of bacteria in 

 particular. It would be easy to demonstrate the formation of 

 soluble ferments related to pepsin and diastase ; the production 

 of soluble and insoluble pigments, and the effect of re-agents upon 

 them ; the relation of vital activity to oxygen as expressed by 

 aerobic and anaerobic germs ; the effect of bacterial growth on 

 various substances, such as blood serum, gelatin, and milk ; the 

 resistance of spores to high temperatures ; the effect of disinfectants 

 and antiseptics; the phenomena of phosphorescence, nitrification, and 

 other equally interesting and instructive features of bacterial life. 

 The habit of close observation and careful differentiation may be 

 cultivated by the parallel study of two species as nearly alike as 

 possible. All this, and more, can be done with bacteria obtainable 

 at any time, from natural waters, from the soil, the digestive tract 

 of mammals and other animals, from milk and various infusions. 

 To impress the mind with the destructive effect of pathogenic 

 forms, a rabbit, or mouse, or guinea-pig may be inoculated with 

 some germ fatal to these animals, but harmless to man. Such a 

 form, fatal to rabbits, is occasionally present in the mouth. The 

 microscopic study of bacteria brings out facts of histo-chemistry, 

 and features of the microscope itself hitherto scarcely known, 

 which should be applied in ordinary histologic work. 



2. There is another class of students who stand in need of such 

 instruction. Much of the preparatory work of the student of medi- 

 cine can and should be done at our higher institutions of learning. 

 For instance, the admirable work done at Cornell University in 

 preparing students for the study of medicine, of which I have per- 

 sonal knowledge, has always tended to push students into the front 

 rank at the medical schools. These have no time to spare to teach 

 students how to dissect well, how to study anatomy or to acquire 

 the methods underlying histologic work and chemical analysis, nor 

 have they the time to teach bacteriology. Yet no one should graduate 

 in medicine to-day who does not know something about the secret 

 working of this miscroscopic world, who cannot reason with it in 

 his practice, or recognize the different forms when a diagnosis may 

 be based upon them. Our biological laboratories may do much to 

 help the medical schools in this direction. The physician will then 

 be equipped with healthier ideas co;icerning the ' germ-theoiy ; ' 



