never looked through an appropriate concave glass. He knows that his companions are able to see better than he does, because they are able to recognise an acquaintance at a much greater distance; but he is unable to conceive the real difference between himself and them, simply because he is only able to reason on the impressions he possesses. He is astounded when he looks through a glass adapted to his vision for the first time. It is a revelation to him. The name applied to them has caused many persons to confuse two distinct classes of impressions, namely, those which we gain of the form of an object by sight, and those which we gain by touch. These are as essentially different from each other as form is from colour. By sight we gain ideas of form as represented in a photograph, and are perfectly able to judge of the form of an object by means of a photograph, which a blind man could not do. The idea of solidity, which is the essential of an impression conveyed by touch, is not a property of sight at all, but depends upon the revival of ideas of objects previously formed, and the effect on the mind of the differences in the impressions received by the two eyes. This may easily be proved by shutting one eye and looking at an object the converse of which is as familiar to the mind as the object itself, as a cameo. With one eye shut it will be impossible to distinguish a cameo from an intaglio, when, by touch, it would be instantly done.

The idea of solidity which is formed on looking at a picture with one eye, all surroundings which might negative this idea being excluded, is due to the revival of previous impressions. The stereoscope shows clearly how two photographs, taken in such a way as to correspond to the impressions which would be received on the

retina of a person looking at the view, give an idea of solidity. That this idea of solidity is a matter of judgment was well shown by a photograph of a marble statue which I had an opportunity of seeing at a late exhibition. It was impossible not to believe that a marble statue was being looked at. The photograph was at the end of a room, and lighted on both sides, and not looked at through glass, nor the ordinary vision interfered with in any way. It will thus be seen that ideas of form gained by the sense of sight are essentially distinct from those derived by touch, and it is as impossible to explain to a congenitally blind man how form can be represented on a plane surface as it is to give him any ideas of colour.

Much surprise has been expressed by many psychologists on finding that a person who has been born blind, having gained his sight by an operation, has failed to recognise objects by sight which he was perfectly cognizant with by touch. This we should expect, as, from what has been previously said, if a cameo represent the impression of touch, a coloured photograph of that cameo will represent the impression of sight.

The following case recorded by Cheselden illustrates this:

'A youth, about twelve years old, who had been born blind, but had obtained his sight by an operation, for some time after tolerably distinct vision had been obtained, saw everything flat, as in a picture, simply receiving the consciousness of the impression made upon his retina; and it was some time before he acquired the power of judging by his sight of the real forms and distances of the objects around him. An amusing anecdote recorded of him shows the complete want

which there is in man of any original or intuitive connection between the ideas formed through visual and through tactile sensations. He was well acquainted with a dog or cat by feeling, but could not remember their respective characters when he saw them; and one day, when thus puzzled, he took up the cat in his arms, and felt her attentively, so as to associate the two sets of cognitions, and then, setting her down, said, 'So, puss, I shall know you another time."

Another very excellent example is recorded by Critchett in the Medico-Chirurgical Transactions,' vol. xxxviii., of a young woman who had been blind from her birth, but had obtained her sight by an operation:

'I found,' says the operator, 'that she was never able to ascertain what an object really was by sight alone, although she could correctly describe its shape and colour; but that after she had once instructed one sense through the medium of the other, and compared the impressions conveyed by touch and sight, she was ever after able to recognise the object without touching it. In this respect her memory was very perfect; I never knew her fail in a single instance, though I put this power frequently to the test of experiment. It was curious to place before her some very familiar object that she had never compared in this way, such as a pair of scissors. She would describe their shape, colour, glistening metallic character, but would fail in ascertaining what they really were until she put a finger on them, when in an instant she would name them, and laugh at her own stupidity, as she called it, in not having made them out before.'

So the remembrance of an impression is in accord

ance with that impression; thus when an afferent impression has been received in the centre for sensory memory, it produces such a modification of the protoplasm of the ganglion cells there, as to allow the complete impression to be brought before the consciousness, when the attention is brought to bear upon the subject. This modification of the protoplasm is permanent, only varying in intensity with time, and constitutes the sensory memory. This diminution in the intensity of the impression is well exemplified in the following way. If a person of average ability, directly after having looked at a picture or a photograph, close his eyes and think of the picture, the remembrance of it will be almost as vivid as the sensory impression itself; if the observer had waited for a minute or two before trying to think of the picture, the remembrance would have been less vivid; in a day feeble, in a week of still less intensity, the remembrance varying with individuals, some being able to have a fair remembrance of the picture, others failing to have the slightest recollection of it.

But, besides the memory of external impressions, there is the memory of internal impressions, that is, the remembrance of thoughts, perceptions, and the other processes of the mind. Thus, when looking at the picture, various ideas usually occur to the observer, and when thinking of it again he remembers these ideas. Every idea and thought which occurs to the mind, depends primarily upon some external influence, however much it may be altered from the original. This forms an important class of impressions, only differing in this, that instead of being received and elaborated by the sensory nerves and their centres, and conducted

upwards to the centres for sensory memory, they are originated by the higher parts of the brain, and conducted downwards to the centre.

Thus, there are two divisions of sensory memory:

1. That memory which consists of the .modification of the protoplasm of the cells of the centre, produced by sensory impressions as received.

2. That memory which consists of the modification of the protoplasm of the cells of the centre, produced by impressions of thoughts and ideas based on sensorial impressions.

Our knowledge of the external world is all based on impressions conveyed to the brain by the sensory nerves, which convey a larger number of impressions than are included under the names of the special senses, sight, hearing, smell, touch, and taste; thus, we also receive impressions of the condition of the muscles, temperature, visceral and pathological sensations.

No impression which has ever been brought before the mind, whether originated by any object in the external world, or by the action of any of the faculties, is ever entirely lost.

This necessarily follows, assuming the process of memory described above to be true, namely, that memory consists of the modification of the protoplasm of the receptive cells produced by an impression, the intensity of the modification only diminishing with time. The change in the protoplasm which constitutes memory can only take place in cells having an original construction fitted for the purpose, in the same way as only the cells of the retina can respond to light. All cells undergo modifications when submitted to certain influencing conditions; but these modifications do not