it would at once be evident that they would be able to see a larger portion of a distant object than we do with two ordinary eyes, and this would produce a more powerful stereoscopic effect. We see from the figure, that in looking at the ball K, the eyes would only see the section

aca, but, with the help of the instrument, they would see the larger section, b a c a b, because the rays from b are reflected by the mirror s1 upon the mirror s. Looking at a landscape in this manner it will seem to have

been brought nearer, because it is only when objects are near to us that we see so large a portion of their surface as we do by means of this instrument; and since the apparent size of the objects remains the same, the impression is produced of a model, in which the different bodies stand out in high relief.

The study of stereoscopic phenomena has also enabled us to explain luminosity. Those bodies are luminous which can reflect light, but are not perfectly smooth and level. Water, when quite smooth, reflects perfectly, but is not luminous, becoming so as soon as its surface is ruffled by ripples. Silk is luminous because each par

ticle reflects light, though altogether they do not form a reflecting surface. Now it has been observed that if two pictures are viewed through the stereoscope, one of which is white and the other black, as in fig. 45, this gives rise to an impression of luminosity. This phenomenon has been explained in the following manner. A given point upon a luminous surface can never appear equally bright to both eyes, because it does not reflect the same amount of light in every direction. We know this, in a general way, from the fact that such luminosity changes its position with the movement of the head. It is to this

peculiarity that luminosity, in a great measure, owes its existence. Now the same effect is produced by looking at the two pictures, one of which appears black to one eye, and the other white to the other eye. They do not combire so as to form an equal tint of grey, but a struggle ensues between the fields of vision, the colour seeming to be first black, then white, and producing the sensation of luminosity. The change from dark to light upon the same point of the retina, produces an unpleasant quivering or twinkling light, which is caused by weariness. But, on the other hand, the alternating changes from dark to light in both eyes, which produces luminosity, is a pleasant sensation. In this case no unpleasant irritation of the retina takes place, such as is caused by a flickering light, but our attention moves from one field of vision to the other, and it is the alternation, apparently, which pleases us. The pleasure which luminous bodies give, and which it is natural for all men to feel, only lasts while the luminous body transmits those changing lights to the eye; it ceases as soon as the surface becomes perfectly smooth, and, as a looking-glass, reflects to us the naked truth.

It is very interesting to observe how faithfully this luminosity is reproduced in photographic stereoscopes. The gleam of the setting sun upon the rippled surface of the sea, the brightness of columns of marble, appear in the pictures just as they do in reality, and yet, when we look at each single picture with the unaided eye, we can only see the dark and light spots, which could never give the impression of luminosity. But when we look closely we see that many spots in the one picture are dark, which

are light in the other. It is the combination of the two which produces luminosity.

The struggle between the fields of vision, which has been mentioned above, is still more remarkable, if, instead of black and white, we choose two colours, blue and red for instance. We do not then see a single mixed tint, as we might be led to suppose, but a hazy uncertain passage from one colour to the other, which wanders hither and thither. Here and there a mixed tint is seen, and the whole has a somewhat luminous appearance.

The struggle between the fields of vision is also called into action, if two differently coloured glasses are placed before the eyes, when looking at a white surface. The interchange of colours is quite irregular, one colour appearing in the midst of the other without any rule. Some experimenters, however, assert that they see one colour or the other at will, from which it would appear that we are able to give special attention first to the image on the right retina and then to that on the left.

The union of the two retinal pictures into a single picture in relief still remains one of the most wonderful phenomena of the senses, in spite of all these observations, which some regard as quite a sufficient explanation. We must be satisfied for the time in fixing the physical and physiological conditions of these phenomena. The representation of the material world is a distinctly mental act which takes place in the brain, and as such will, for a long time, escape scientific research.

CHAPTER IX.

Optical Illusions--Apparent Size of the Moon—Intuition—Illusions of the Sense of Colour-Mental and Corporeal Vision.

HOWEVER perfect may be the optical apparatus with which nature has provided us, there are cases in which we see things differently to what they are in reality. Such phenomena we call optical illusions, and they are the more interesting because they give us a closer insight into the act of perception.

We can judge with tolerable accuracy as to whether two lines are parallel to each other or not. Zöllner, however, has remarked that we are subject to a curious. deception if the parallel lines are crossed by short oblique lines which slant inwards as in fig. 46. The lines 2 and 3 appear at first sight to widen downwards, the lines 1 and 2 upwards, and yet they are exactly parallel to each other when measured. The oblique strokes on the lines 2 and 3 would intersect each other if extended downwards, and the result is that the Enes themselves look as if they would intersect each other if extended upwards. Thus the apparent convergence of the lines always has an opposite direction to that of the oblique strokes.