Objects , Advantages, And Pleasures Of Science.

li « lit- But another discovery Avas , about fifty years afterwards , made by Mr , Dollond , —that , by combining different kinds of ulass in a compound magnifier , the difference may ho greatly coi'rected ; and on this princip le he constructed his telescopes . It is found toothat the different natural

, magnifiers of the eye are combined upon a principle of the same kind . Thirty years later , a third discovery Avas made by Mr . Blair , of the greatly superior effect which combinations of different liquids have hi correcting the imperfection ; andmost

, Avonderful to think , when the eye is examined , Ave find it consists of different liquids , acting naturally upon the same p rinciple Avhich Avas 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 Ave have once ascertained b y experiment , that light is bent in a certain

Avay when it passes through transparent bodies . Now birds flying in the air , and meeting with many obstacles , as branches ancl leaves of trees , require to have their eyes sometimes as fiat as possible for protection ; but sometimes as round as possible , that they see the small objectsflies

may , , and other insects , Avhich they are chasing through the air , and Avhich they pursue with the most unerring certainty . This could onl y be accomplished by giving them a power of suddenly changing tho form of their eyes . Accordingly there is a set of hard

scales placed on the outer coat of their eye , round the place Avhere the light outers ; and over these scales arc drawn the muscles or fibres by Avhich motion is jtoiimmiiicated ; so that , by acting with these musclesthe bird can the scales

, press * i"l squeeze tho natural magnifier of the <; . )' e into a round shape when it ivishes to lo jW an insect through the air , and can ^ lax the scales in order to flatten the eye % 'aiu Avhen it would see a distant object

, , ' * move safel y through leaves and twigs . .- '" s poAver of altering the shape of the ' eye s Possessed by birds of prey in a very re"I'lihublu degree . They ' can thus see the

smallest objects close to them , and can yet discern larger bodies at vast distances , as a carcase 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 AA'iping the glass of the instrument , as it

were—ancl also for protecting it , Aidiile rapidly flying through the air , and through thickets , Avithout hindering the sight . Birds are , for these purposes , furnished with a third eyelid , a fine membrane or skiuAvhich is constantly moved very

, rapidly over the eyeball by two muscles p laced in the back of the eye . One of the muscles ends in a loop , the other in a string Avhich goes through the loop , and is fixed in the corner of the membrane , to pull it back-Avard and fonvard . If you Avish to draw a

thing toAvards any place Avith the least force , you must pull directly in the line between the thing and the place ; but if you irish to draw it as quickly as possible , ancl AA'ith the most convenience , and do not regard the loss of force , you must pull it obliquely , by draAving it in tAvo directions ' at once . Tie a string to a stone , and chaAV it straig ht towards you with one hand ;

then , make a loop on another string , and running tho first through it , draw one string in each hand , not toivards you , but side-Avays , till both strings are stretched in a straight line , you Avill see IIOAV much more easily the stone moves quickly than it did before when pulled straight forward .

Again , if you tie strings to the tivo ends of a rod , or slip of card , in a running groove , ancl bring them to meet ancl pass through a ring or hole , for every inch in a straight line that you draw both together bcloAV the ring ,. the rod Avill move onward two .

NOAV this is proved , by mathematical reasoning , to be the necessary consequence of forces applied obliquely ; there is a loss of poAver , but a great gain in velocity . 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 , as the tivo strings are by the hands in the cases we have been supposing . ¦ A third eyelid of the same kind is found in the horse , and called the lin-w ; it is moistened Avith a pulpy substance for mucilage ) to take hold of the dust on the eyeball , and Avipe it clear oh : so that the