On this page
-
Text (1)
-
Untitled Article
Note: This text has been automatically extracted via Optical Character Recognition (OCR) software. The text has not been manually corrected and should not be relied on to be an accurate representation of the item.
-
-
Transcript
-
Note: This text has been automatically extracted via Optical Character Recognition (OCR) software. The text has not been manually corrected and should not be relied on to be an accurate representation of the item.
Additionally, when viewing full transcripts, extracted text may not be in the same order as the original document.
Untitled Article
the following considerations . Two things were wanted in the eye , which were not wanted , in the same degree at least , in the telescope 3 these were , the adaptation of the organ ( 1 ) to different degrees of light ; and ( 2 ) to
the diversity of distance at which objects are viewed by the naked eye ; riz . from a few inches to miles . These are difficulties which do not present themselves to the telescope-maker . He wants all the light he can get ,
and he never directs his instrument to objects near at hand . In the eye both these cases were to be provided for , and for this purpose an appropriate mechanism is introduced . In order to exclude an excess of
light , and to render objects visible under obscurer degrees of it , when no more can be had , the aperture of the eye through which the light enters , is so formed as to contract or dilate itself , for the purpose of admitting a greater or less number of rays at the same time . Hence the chamber of
the eye is like a camera obscura , which , when the light is too small , can enlarge its opening ; when too strong , can contract it , and this without any other assistance than that of its own exquisite machinery : it should , however , be added , that the pupil of the eye , under all its different
dimensions , retains its exact circular shape . This structure is extremely artificial . 11 Let an artist , " says I > r . Paley , " only try to execute the same . He will find that his threads and strings must be disposed with great consideration and contrivance , to make a
circle which shall continually change w diameter , yet preserve its form . This is done in the eye by fin application of fibres , that is , of strings , similar hi their position and action to what an artist would and must em-Ptoy > if he had the same piece of workmanship to perform . "
To suit the same organ to the perception of objects that lie near at to" **! and to those at a considerable ^ taricje , is another difficulty to be ^ rmountedv According to the prints of optics , this could not be ao without the ok-gan itself under-U ?* an alteration and receiving an ^ J ^ tment , that might correspond ^ the different inclination to one J ^ ther under which the rays of light Dla j ^ * k &ay » issuing from points Vl *** l at a small distance from the
Untitled Article
eye , and which must enter the eye in a diverging order , cannot , by the same optical instrument in the same state , be brought to a point , that is , cannot be made to form an image in the same place , with rays proceeding
from objects situated at a greater distance , and which rays arrive at the eye in directions nearly , and physically speaking , parallel . It requires a rounder or more convex lens to do it . The point of concourse , that is *
the point behind the lens where the rays meet 9 must be on the retina , or the vision is confused ; yet this point is carried farther back , when the rays proceed from a near object , than when they are sent from one that is remote . This matter would be
managed in a telescope by alteriijff the distances of the lenses by means of screws or other contrivances . But in the eye the alteration is effected by the action of certain muscles , by means of which , whenever the eye is directed to a near object , three
changes are produced in it at the same time , all contributing to the adjustment required . The cornea is rendered more round or prominent , the crystalline lens underneath is pushed forward , and the axis of vision , as the depth of the eye is called , is
elongated . These changes in the eye vary its power over the rays of light in such a manner and degree , as to produce exactly the effect which is required , viz . the formation of an image upon the retina , whether the rays
coine to the eye in a state of divergency , which is the case when the object is near the eye * or when they come parallel to one another , as is the case when the object is placed at a distance . Surely nothing can be more decisive of contrivance than this . The
most secret laws of optics must have been known to the Author of a structure endowed with such a capacity of change . 41 Observe , says Dr . Paley , a " newborn child lifting up its eye-lids , and it will be found that th £ anterior part of the two globes are constructed
upon strict optical jprinciples . They are , for the purpose of forming an image by refraction , composed of parts executing different offices ; one part having fulfilled its office upon the pencil-light , delivers it over to the action of another part , that to a third , and so on . The progressive
Untitled Article
Natural Theology . No . IV . — -The Eye . 235
-
-
Citation
-
Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), April 2, 1815, page 235, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct1759/page/35/
-