OBJECTS, PLEASURES, AND ADVANTAGES OF SCIENCE. 



xvii 



to consist of differently-coloured parts, which are differently bent in passing through transparent substances, 

 so that the lights of several colours come to a point at different distances, and thus create an indistinct 

 image at any one distance. This was long found to make our telescopes imperfect, insomuch that it 

 became necessary to make them of reflectors or mirrors, and not of magnifying glasses, the same difference 

 not being observed to affect the reflection of light. But another discovery was, about fifty years afterwards, 

 made by Mr. Dollond that, by combining different kinds of glass in a compound magnifier, the difference 

 may be greatly corrected ; and on this principle he constructed his telescopes. It is found, too, that the 

 different natural magnifiers of the eye are combined upon a principle of the same kind. Thirty years later, 

 u third discovery was made by Mr. Blair, of the greatly superior effect which combinations of different 

 liquids have in correcting the imperfection; and, most wonderful to think, when the eye is examined, we 

 find it consists of different liquids, acting naturally upon the same principle which was thus recently found 

 out in optics,by many ingenious mechanical and chemical experiments. 



Again, the point to which any magnifier collects the light is more or less distant as the magnifier is flatter 

 or rounder, so that a small globe of glass or any transparent substance makes a microscope. And this 

 property of light depends upon the nature of lines, and is purely of a mathematical nature, after we have 

 once ascertained by experiment, that light is bent in a certain way when it passes through transparent 

 bodies. Xow birds flying in the air, and meeting with many obstacles, as branches and leaves of trees, 

 require to have their eyes sometimes as flat as possible for protection ; but sometimes as round as possible, 

 that they may see the small objects, flies, and other insects, which they are chasing through the air, and 

 which they pursue with the most unerring certainty. This could only be accomplished by giving them a 

 power of suddenly changing the form of their eyes. Accordingly, there is a set of hard scales placed on 

 the outer coat- of their eye, round the place where the light enters ; and over these scales are drawn the 

 muscles or fibres by which motion is communicated; so that, by acting with these muscles, the bird can 

 press the scales, and squeeze the natural magnifier of the eye into a round shape when it wishes to follow an 

 insect through the air, and can relax the bcales, in order to flatten the eye again, when it would see a distant 

 object, or move safely through leaves and twigs. This power of altering the shape of the eye is possessed 

 by birds of prey in a very remarkable degree. They can thus see the smallest objects close to them, and 

 can yet discern larger bodies at vast distances, aa a carcas* stretched upon the plain, or a dying fish afloat 

 on the water. 



A singular provision ia made for keeping the surface of the bird's eye clean for wiping the glass of 

 the instrument, as it were and also for protecting it, while rapidly flying through the air and through 

 thickets, without hindering the sight. Birds are, for these purposes, furnished with a third eyelid, a line 

 membrane or skin, which is constantly moved very rapidly over the eyeball by two muscles placed in the 

 back of the eye. One of the muscles ends in a loop, the other in a string which goes through the loop, 

 and is fixed in the corner of the membrane, to pull it backward and forward. If you wish to draw a tiling 

 towards any place with the least force, you must pull directly in the line between the thing and the place ; 

 but if you wish to draw it as quickly as possible, and with the most convenience, and do not regard the loss 

 of force, you must pull obliquely, by drawing it in two directions at once. Tie a string to a stone, and 

 draw it straight towards you with one hand ; then make a loop on another string, and running the first 

 through it, draw one string in each hand, not towards you, but sideways, till both strings are stretched in a 

 straight line : you will see how much more easily the stone moves quickly than it did before when pulled 

 straight forward. Again, if you tie strings to the two ends of a rod, or slip of card, in a running groove, 

 and bring them to meet and pass through a ring or hole, for every inch in a straight line that you draw both 

 together below the ring, the rod will move onward two. Now this is proved, by mathematical reasoning, 

 to be the necesary consequence of forces applied obliquely : there is a loss of power, but a great gain in 

 Telocity and convenience. This is the thing required to be gained in the third eyelid, and the contrivance 

 is exactly that of a string and a loop, moved each by a muscle, aa the two strings are by the hands in the 

 case* we have been supposing. 



A third eyelid of the same kind is found in the horse, and called the June ; it is moistened with a pulpy 

 substance (or mucilage) to take hold of the dust on the eyeball, and wipe it clear off; so that the eye is 

 hardly ever seen with anything upon it, though greatly exposed from its size and posture. The swift motion 

 of the haw is given to it by a gristly elastic substance placed between the eyeball and the socket, and striking 

 obliquely, so as to drive out the haw with great velocity over the eye, and then let it come back as quickly. 

 Ignorant persons, when this haw is inflamed from cold, and swells so as to appear, which it never does in a 

 healthy state, often mistake it for an imperfection, and cut it off: so nearly do ignorance and cruelty 

 produce the same mischief. 



If any quantity of matter, as a pound of wood or iron, be fashioned into a rod of a certain length, 

 nay one foot, the rod will be strong in proportion to its thickness; and, if the figure be the same, that 

 thickness can only bo increased by making it hollow. Therefore hollow rods or tubes, of the same length 

 and quantity of matter, have more strength than solid ones. This is a principle BO well understood now, 

 that engineers make their axles and other parts of machinery hollow, and threfore stronger with the 



