PLEASURES OF SCIENCE. 29 



colours come to a point at different distances, and thus create an in- 

 distinct image. This was long found to make our telescopes imper- 

 fect, insomuch that it became necessary to make them of reflectors 

 or mirrors, and not of magnifying glasses the same difference not 

 being observed to aifr?t their reflection But another discovery was 

 about fifty yearv afterwards made by Mr. Dollond, that by combining 

 different kinds of glass in a compound magnifier, the difference maybe 

 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, a 

 third discovery was made by Mr. Blair, of the greatly superior effect 

 which combinations of different liquids have in correcting the imper- 

 fection ; and, most wonderful to think, when the eye is examined, we 

 find it consists of different liquids, acting naturally upon the same prin- 

 ciple 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 smaller or rounder, so that a small globe 

 of glass or any transparent substances 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 experi- 

 ment, that light is bent in a certain way when it passes through 

 transparent bodies. Now 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 scales, 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 see the smallest objects close to them, and can yet 

 discern larger bodies at vast distances, as a carcass stretched upon the 

 plain, or a dying fish afloat on the water. 



A singular provision is 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 fine 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 thing towards any 

 place with the least force, you must pull directly in the line between 





