October 9, 1885.] 



SCIENCE, 



309 



circumstances, and these variations seem often in 

 a singular way to conform to what they would be 

 like if the retinal excitements acted on the mind 

 by suggesting space determinations learned in some 

 other way. Even should the retinal tracts have 

 innate feehngs of extension of their own, the vari- 

 ations in question force us to admit that the innate 

 extensions etc. , are often overpowered by the sug- 

 gestion of other and different ones. Thus the 

 nativistic school of explanation is replaced by the 

 ' empuistic ' school, as Helmholtz calls it. The 

 experiences, whose suggestions prove themselves 

 to be so much more powerful than anything else, 

 are, for these authors, on the whole, experiences of 

 motion. The movements of the eyeball are the 

 deus ex machina which shall solve all riddles. The 

 excessive intricacy and delicacy of the facts to be 

 interpreted can be estimated by the differences of 

 opinion that still exist among the interpreters — 

 many of them men of as great ability as our 

 century has shown in any intellectual field. Dr. 

 Lipps now rushes into their midst, and deals blows 

 against the whole movement-theory that ought 

 really to warm the hearts of its doubters. What- 

 ever it be that measm*es off the field of view and 

 estabhshes du-ections and distances between the 

 impressions we get from retinal points, it is, ac- 

 cording to him, neither muscular sensation nor 

 feeling of innervation, — it must be something else. 

 Let us say here, that however it may fare with Dr. 

 Lipps's positive theories, this critical onslaught of 

 his is a permanent achievement from which it will 

 be hard for the muscle-theory to recover. ' Feel- 

 ings of movement ' and ' unconscious inferences ' 

 have too long run riot and had it their own way in 

 the philosophy of vision. Being more or less hypo- 

 thetical entities, one may construct very much 

 what one pleases with them, and hitherto they 

 have turned a deaf ear to critics. The champions 

 of the feehngs of movement can, however, hardly 

 ignore Dr. Lipps's manner of caUing their proteges 

 to account. 



Dr. Lipps's own theory is nativistic to the extent 

 of admitting that if ever retinal impressions are 

 discriminated at all, their difference will (by an in- 

 exphcable law) appear as a difference of position. 

 It is however ' empiristic ' first, in that it assumes 

 that no discrimination would occur at all unless 

 differently colored extended objects were what ex- 

 cited the retina in the first instance ; and second, in 

 that it makes the average extension of the objects 

 determiae the extension at which the various ex- 

 cited retinal poiats shall appear to us apart. Ad- 

 jacent points everywhere on the retuia are more 

 likely to have a portion of one object than the 

 boundary of two objects cast upon them. Distant 

 points are more likely to be excited by different 



objects than by the same object. Distant points 

 will tend then rather to be discriminated ; ad- 

 jacent points rather to remain fused together. 

 The object-experiences of intermediate points will 

 have helped partly to separate them, partly to keej) 

 them together. The author seems to think that 

 with a greater tendency of two points to be dis- 

 criminated will go a feeling of their greater, and 

 with a lesser tendency, a feehng of their lesser, 

 distance apart — a point which he has not made 

 theoretically sufficiently clear. 



The tendency to be fused together until discrimi- 

 nated is for Dr. Lipps fundamental. That the 

 borders of the blind spot should give images that 

 are fused, and run into each other, that are with- 

 out breach of visual continuity between them, is 

 nothing peculiar. Every part of the retina is 

 similarly continuous with every other, even distant, 

 part. We see then space continuous over the 

 blind spot. How much we see there is determined 

 by the general law of discrimination. The two 

 borders of the spot receive images sometimes of 

 the same, sometimes of different objects, and the 

 balance of their tendencies to fuse and separate is 

 what will determine their apparent distance apart. 

 A close study of the actual phenomena of the 

 blind spot is apparent in this section. 



The section on the perception of the thu'd 

 dimension, depth, or distance, is properly an exiDan- 

 sion of Ferrier's commentary on Berkeley. Berke- 

 ley said we cannot see distance. Ferrier, the meta- 

 physician, said we can see space only between two 

 things both of which we see. We cannot see our 

 own eye ; ergo we cannot see the space bet^veen it 

 and anything else. But such space is what is 

 meant by distance ; ergo we cannot see distance. 

 Dr. Lipps enforces this by the most remorseless 

 logic, denying that there is any properly so called 

 visual perception of the third dimension at all. 

 There is merely a conceptual knowledge of it. He 

 makes a brave attempt to explain away the appar- 

 ently direct sensational character of this knowl- 

 edge, as when we look, for example, into the 

 stereoscope ; and he makes a heavy attack on 

 Stumpf as the ablest advocate of a dh-ect feeling of 

 depth. He carries the discussion to a j)oint, as it 

 seems to us, where it becomes largely a matter of 

 words. To admirers of Berkeley, however, it may 

 be said, that nowhere has the origmal negative 

 Berkeleian doctrine about distance received any- 

 thing like such able support as tliis. 



In the essay on inusical discord, our author re- 

 verts to the old-fashioned theory of a subtle sense 

 for the incongruity of the rates of vibrations of the 

 notes simultaneously heard. He shows by an in- 

 teresting experiment how hard it is to hear one 

 rhythm made outside of us, and to carry on a differ- 



