28 



OPTICAL INSTRUMENTS. 



correct for one case, and the distance 

 of the image or focal point is varied, 

 the divergency of the rays will alter, 

 and the dispersion will be either too 

 much or too little corrected. The pro- 

 portion of the foci of these two lenses 

 should be as 2 to 3, to produce the 

 largest field and the least distortion. 



(45.) The eye-pieces employed to 

 produce an erect image for terrestrial 

 telescopes admit of numerous arrange- 

 ments : the simplest was shown in fig. 1 6, 

 composed of three similar lenses placed 

 at equal distances ; but as there is no 

 field-lens in this construction, the view 



is limited, though the chromatic dis- 

 persion is in part corrected by the 

 middle lens, and may be totally de- 

 stroyed by using an eye-glass of shorter 

 focal length than the other two, and at 

 a less distance from the middle glass. 

 Another combination may be produced 

 by two glasses so placed that the lens 

 which receives the inverted image 

 formed by the object-glass shall form 

 an image on the other side, inverted 

 with respect to the first, but erect with 

 the object, and this image may be viewed 

 with an eye-glass. This construction 

 is shown in jig. 30. Let i be the in- 



Fig. 30. 



verted image formed by the object-glass, 

 o a a pencil of light from the centre of 

 the object-glass falling on the lens A ; 

 this pencil will be dispersed after re- 

 fraction, and a r will be the path of the 

 red rays, and a v of the violet .; at the 

 focal point e an erect image will be 

 formed, and may be viewed by the lens 

 E. Now the red rays at r falling on 

 this lens will be less refracted than the 

 violet at v, and will cross the axis in R, 

 while the violet v crosses the axis 

 at V ; hence the dispersion will be 

 greatly increased both by reason of the 

 spherical aberration, and their greater 

 refrangibility, so that objects seen 

 through this eye-piece will be more 



fringed with colours than in any other 

 construction. 



(4G.) The last construction might 

 be improved by using two lenses dis- 

 posed in a similar manner to the nega- 

 tive eye-piece (44,) instead of the lens 

 E. These would enlarge the field and 

 correct the dispersion ; but as the sphe- 

 rical aberration is very great, two lenses 

 should be used in place of the lens 

 A, in order to make the combination 

 complete ; thus a four-glass eye-piece 

 would be formed that is perfectly cor- 

 rected, if duly proportioned distances 

 and foci are used. This is accomplished 

 in the following combination (fig. 31.) 

 where the lenses are in the order of the 



Fig. 31. 



letters A B C D beginning with the 

 lens next the object-glass, and the dis- 

 tances, &c. of each lens are as follows : 



1.18, B C = 1.83, C D = 1.105. Stops 

 should be placed between the lenses 

 A and B, and a larger one between C 

 focal length of A=l| = B = 2J, C = 2 and D ; these stops are to prevent any 

 an dD = l, and their distances AB = 2, false light from passing through the 



lenses to the eye. The more black stops 

 there are introduced into a telescope, 

 provided they do not hinder the pencils 

 of light proceeding from the object, the 

 better will the instrument perform. 



BC = 3|,CD = 2f. This eye-piece will 

 be nearly free from chromatic dispersion 

 and aberration. In a very good eye- 

 piece of Ramsden's, the focal lengths 

 were found to be of A.= . 0775, B= 1.025, 

 C = i.oi, D = .79 ; the distances A B = 



