APPARENT MAGNITUDE. 



small proportion to the distance, it is shown by the principles of 

 geometry that the angle I/ E M' is less than the angle L E H in the 

 same proportion as E M' is greater than E M, and that the angle 

 L" E M" is less than L E M in the same proportion as E 3d." is 

 .greater than E M. 



38. Nothing can be more calculated to excite our wonder and 

 admiration than the distinctness of our perception of visible 

 objects, compared with the magnitude of the picture on the retina, 

 from which immediately we receive such perception. 



39. If we look at the full moon on a clear night, we perceive 

 with considerable distinctness by the naked eye the lineaments of 

 light and shade which characterise its disk. Now let us consider 

 only for a moment what are the dimensions of the picture of the 

 moon formed on the retina, from which alone we derive this 

 distinct perception. 



The disk of the moon subtends a visual angle of half a degree, 

 and consequently, according to what has been explained, the 

 diameter of its picture on the retina will be ^th part of an inch, 

 and the entire superficial magnitude of the image from which we 

 derive this distinct perception is only the smooth part of a square 

 inch ; yet within this minute space, we are able to distinguish a 

 multiplicity of still more minute details. We perceive, for 

 example, forms of light and shade, whose linear dimensions do 

 not exceed one-tenth part of the apparent diameter of the moou, 

 and which therefore occupy upon the retina a space whose 

 diameter does not exceed the goosoooth part of a square inch. 



40. To take another example, the figure of a man 70 inches 

 high, seen at a distance of 40 feet, produces an image upon the 

 retina the height of which is about one-fourteenth part of an inch. 

 The face of such an image is included in a circle whose diameter 

 is about one-twelfth of the height, and therefore occupies on the 

 retina a circle whose diameter is about the ^th part of an inch ; 

 nevertheless, within this circle, the eyes, nose, and lineaments are 

 distinctly seen. The diameter of the eye is about one-twelfth of 

 that of the face, and therefore, though distinctly seen, does not 

 occupy upon the retina a space exceeding the 400 ^ 000 th of a 

 square inch. 



If the retina be the canvas on which this exquisite miniature is 

 delineated, how infinitely delicate must be its structure, to receive 

 and transmit details so minute with such marvellous precision ; 

 and if, according to the opinion of some, the perception of these 

 details be obtained by the retina feeling the image formed upon 

 the choroid, how exquisitely sensitive must be its touch ! 



41. 3. SUFFICIENCY OF ILLUMINATION. 



It is not enough for distinct vision that a well-defined picture of 



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