THE RAMSDEN EYE-PIECE. 



51 



FIG. 32. 



refractive power is less than at the margin, so that 

 spherical error of the eye-lens constitutes a nearly 

 perfect balance to the chromatic dispersion of the 

 field-lens, and the blue and red rays V and I" emerge 

 sensibly parallel, presenting, in consequence, the perfect 

 definition of a single point to the eye. The same reason- 

 ing is true of the intermediate colours and of the other 

 pencils. The eye-glass thus constructed not only brings 

 together the images b' b', r r r', but it likewise has the 

 most important effect of rendering them flat, and at once 

 correcting both chromatic and spherical aberrations. 



The Huyghenian eye-piece described, has served 

 the purpose of illustrating the optical 

 effects of this part of the instrument ; 

 but when it is required to measure the 

 magnified image, we use the eye-piece 

 invented by Bamsden, and called by 

 him the micrometer eye-piece. The 

 arrangement will be readily understood 

 upon reference to fig. 32. The field- 

 glass having its plane face turned to- 

 wards the object, so that the rays from the object are 

 made to converge immediately in front of the field-glass; 

 and here is placed a plane-glass, on which is engraved 

 divisions of l-100th of an inch or more. The markings 

 of these divisions come 

 into focus, therefore, at the 

 same time as the image of 

 the object, both being dis- 

 tinctly seen together. The 

 glass with its divisions is 

 shown in fig. 33, on which, 

 at A, are seen some magni- 

 fied grains of starch. Thus 

 the measure of the magni- 

 fied image is given by mere 

 inspection ; and the value 

 of such measurements, in 

 reference to the real object, when once obtained, is 

 constant for the same object-glass. 1 



(1) It was affirmed by Ross, that if the achromatic principle were applied 

 E 2 



FIG. 33. 



