630 THE SENSES. 



any part of the landscape, objects at a certain distance only are dis- 

 tinctly seen ; all others, situated within or beyond this distance, are 

 obscure or imperceptible. This is necessarily the case, since a lens or 

 system of lenses can bring to a focus at one spot only those rays which 

 strike its anterior surface within a certain degree of divergence. The 

 formation of a visible image at the desired spot depends entirely upon 

 the refracting power of the lenses being such, that all the rays diverg- 

 ing from a particular point of the object shall be again brought to an 

 exact focus at the plane where the image is to be perceived. If the object 

 be placed at an indefinite distance near the horizon, or if it be one of 

 the heavenly bodies, the rays emanating from any one point of such an 

 object reach the telescope under so slight a degree of divergence that 

 they are nearly parallel; and, on suffering refraction, they will be 

 brought to a focus at a short distance behind the lens. But if the 

 object be less remote, the rays emanating from it strike the lens under 

 a higher degree of divergence. The same amount of refractive power, 

 therefore, produces a less rapid convergence than in the former case, 

 the rays are consequently brought to a focus only at a greater distance 

 behind the lens. To provide for this difficulty, the spy-glass is pro- 

 vided with a sliding tube, by which the distance of the eye-piece from 

 the object-glass may be shifted at will. For the examination of remote 

 objects, the eye-piece is pushed forward, so as to bring into view the 

 image formed at a short distance behind the lens ; for the examination 

 of near objects it is drawn backward, to receive the image placed 

 farther to the rear. This is the accommodation of the spy-glass for 

 yision at different distances. 



A similar necessity exists in the optical apparatus of the eye. If 

 one eye be covered, and two long needles placed vertically in front of 

 the other, in nearly the same linear range, but at different distances- 

 one, for example, at eight, and the other at twenty inches from the 

 eye it will be found that they cannot both be seen distinctly at the 

 same time. When we look at the one nearer the eye, so as to perceive 

 its form distinctly, the image of the more remote one becomes con- 

 fused; and when we see the more distant object in perfection, that 

 which is nearer loses its sharpness of outline. 



The same thing may be made evident by stretching in front of the 

 eye, at the distance of seven or eight inches, a plain gauze veil, or other 

 woven fabric formed of fine threads, with tolerably open meshes, so 

 that objects beyond may be readily visible through its tissue. The ob- 

 server, in using a single eye, may fix at will either the threads of the 

 veil, or the more distant objects placed beyond it ; but they alternate 

 with each other in distinctness, like the two needles in the experiment 

 described above. At the time when the threads are sharply defined, 

 other objects are indistinct ; and when the eye is fixed upon the more 

 distant objects, so that they are perfectly delineated in the field of 

 vision, the threads of the veil become almost imperceptible, and hardly 

 interfere by their presence with the images seen beyond. 



