198 



• KNOWI.EDGE . 



[Sept. 5, 1884. 



It was a question whether the arc lamps would answer 

 for this purpose in the smoky atmosphere. For the 

 first experiments, arc lamps of 3,000 and 1,000 candles 

 were used, with the positive carbon in the lower 

 holder. The effect was brilliant, yet the light did not 

 penetrate the white smoke cloud which collects at the 

 upper wall immediately after the shot But as the smoke 

 settles within ten minutes, it was thought advisable to 

 acquiesce in this interruption of a few minutes, and to use 

 smaller lamps of 350 candles, which proved quite efficient. 

 Of these, there are ten in use, with about 10,000 ft. of lead 

 cable, the cable being partially elastic, as the lamps with 

 their wires have to be removed when the blasting is to take 

 place. The lamps were originally supplied with hexagonal 

 lanterns with obscured glass to protect the eyes of the 

 miners. The glasses were, of course, soon broken, but no 

 complaints are said to have been made about the naked 

 electric lights. The proprietors of the mine have decided 

 upon an extension of the installation. 



OPTICAL RECREATIONS. 

 By a Fellow of the Royal Astronomical Society. 



(Continued from page 130.) 



IN all that has preceded, we have, so to speak, taken 

 sight for granted. Before, however, proceeding to 

 examine the phenomena which essentially depend upon the 

 very way in which we see at all — e.g., those of binocular 

 vision and the like — it will be necessary to enter into a 

 little elementary explanation of the structure of the eye 

 itself. The student who may wish to follow this subject 

 out in detail must consult some of the larger textbooks on 

 Human Anatomy. It will conduce to a better appreciation 

 of our description if he will obtain a fresh sheep's or 

 bullock's eye from the butcher. 



We shall remark in the outset that the eye-ball is very 

 nearly globular, but the front part (Fig. 25) is more convex 

 than the other portion. Now, excepting this front pro- 

 tuberant part, the eye will be observed to be covered by a 



Cliomi'd 



Crustalhn cLcns- 

 Cornea 



Ins 



OpticXcrve 



ei'Olicr 



Fig. 25. 



thick white membrane called the sclerotic coat, which is 

 popularly known as " the white of the eye." The front of 

 the eye is enclosed by a thick, strong, glassy membrane 

 called the cornea, which suggests the idea of a watch-glass. 

 The light quite obviously passes into the eye through the 

 cornea. From the circle of junction of the cornea and 

 the sclerotic, a kind of opaque coloured plate, with 

 a hole io the middle, separates the front clear part 

 of the eye from the main portion of the interior of the 

 eyeball. It is called the iris, and it is the colour of the iris, 

 which is spoken of as that of the eye, when we talk about 

 blue eyes, grey eyes, brown eyes — and so on. The hole in 

 the centre of this iris is known as the pupil. This appears 

 jet-black, because we look through it at the dark interjor of 

 the eye-ball. The function of the pupil is to admit 1 ight, 

 and further to regulate the amount of light so admitted. 

 It contracts in the presence of a bright light, and expand s 



in partial darkness — a fact of which the reader may satisfy 

 himself by standing before a looking-glass in a dim light, 

 and watching the pupils of his own eyes when a candle or 

 lamp is suddenly brought near them. The same result may 

 be attained in daylight by shutting the eyes close and 

 opening them suddenly before a mirror. This is all that 

 we can make out from the exterior of the eye. We must 

 dissect it (preferably under water) if we wish to under- 

 stand its internal structure. Doing so, we shall find that 

 the sclerotic is lined by the choroid membrane, of a dart 

 brown or black colour ; as is that again in turn by the 

 retina : the latter being nothing but an expansion of the- 

 optic nerve after it enters the eye from the brain. The 

 choroid coat at the front of the eye splits into two layers, 

 one going to form the iris already spoken of, the other 

 taking the shape of a kind of pleated curtain. To this is 

 attached the crystalline lens — an absolutely transparent, 

 double-convex lens, between which and the cornea lies a 

 clear liquid called the aqueous humour , while the greater 

 portion of the cavity of the eye — that behind the crystalline 

 lens — is filled with a transparent, gelatinous substance- 

 known as the vitreous humour. Our figure above should" 

 make this short description intelligible. From it we gather 

 that the eye consists in effect of an optical instrument, like- 

 the camera obscura (Fig. 2, Vol. V., p. 306, and Fig, 20, 

 Vol. VI., p. 46), in which an image formed by the refrac- 

 tion through a combination of lenses of the rays of light 

 emanating from various external objects is projected on to 

 the back of the eye, and being there received on the retina, 

 is conveyed by the optic nerve to the brain. That such an 

 image is formed on the retina, the student may satisfy him- 

 self by carefully scraping the back of a bullock's eye (under 

 water) until it becomes thin enough ; when, on holding the 

 eye u[) with the cornea directed to the window, or a well- 

 lighted landscape, a charming little picture of the object 

 towards which it is directed may be seen on the semi- 

 transparent screen to which the hinder part of the eye 

 has been reduced. It -will be noted that this image is 

 inverted, and a large amount of unprofitable discussion has 

 arisen as to why we do not see external objects upside 

 down (as though we had another eye at the back of the first 

 one to view the image formed by that !). The fact is that 

 we do not see anything in the eye, but something wholly 

 external to it. For it is the brain, after all, that sees, and 



Fig. 26. 



not the eye. Sever the optic nerve, and utter and absolute 

 blindness instantly supervenes ; albeit the eye, as an optical 

 instrument, remains intact In the sensorium we refer any 

 object to the direction in which the light from it reaches 

 the eye. If we have to raise the head, as to view the 

 zenith, we say the object regarded is above us or up ; if we 

 have to depress the head, as in looking at the ground, 

 we say that the object is beneath us or down. The 

 so-called " mystery " of inverted vision is then, in reality, 

 no mystery at all. Various illustrations may be found of 

 this reference by the brain (through the optic nerve) of 

 external objects to the direction whence the light they emit 

 or reflect enters the eye. For example, press the comer 

 of the eye with the finger, and a patch of light will appear 

 as existing in the direction of the pressure. Look at the 

 bright sky, preferably through an astronomical telescope. 



