562 PHILOSOPHICAL TRANSACTIONS. [aNNO 1779- 



This micrometer is applied to the erect eye tube of a refracting telescope, and 

 is placed in the conjugate focus of the first eye-glass: hence arises its great 

 superiority to the object-glass micrometer. It has been before (observed, that if 

 a micrometer is applied at the object-glass, the imperfections of its glass are 

 magnified by the whole power of the telescope; but in this position, the image 

 being considerably magnified before it comes to the micrometer, any imperfection 

 in its glass will be magnified only by the remaining eye-glasses, which in any 

 telescope seldom exceeds 5 or 6 times. By this position the size of the micro- 

 meter-glass will not be the 100th part of the area which would be required if it 

 was placed at the object-glass; and, notwithstanding this great disproportion of 

 size, which is of great moment to the practical optician, the same extent of 

 scale is preserved, and the images are uniformly bright in every part of the field 

 of the telescope. 



Fig. 10 represents the glasses of a refracting telescope; xy the principal 

 pencil of rays from the object-glass O; tt and uu the axes of 2 oblique pencils; 

 a the 1st eye-glass; m its conjugate focus, or the place of the micrometer; b 

 the 2d eye-glass, c the 3d, and d the 4tli, or that which is nearest the eye. 

 Let p be the diameter of the object-glass, e the diameter of a pencil at m, and f 

 the diameter of the pencil at the eye: it is evident, that the axis of the pencils 

 from every part of the image will cross each other at the point m; and e, the 

 width of the micrometer-glass, is to p the diameter of the object-glass, as ma 

 is to go, which is the proportion of the magnifying power at the point m ; and 

 the error caused by an imperfection in the micrometer-glass placed at m, will be 

 to the error had the micrometer been at 0, as m is to p. 



Fig. 9, represents the micrometer; A a convex or concave lens, divided into 

 2 equal parts by a plane across its centre; one of these semi-lenses is fixed in a 

 frame b, and the other in the frame e, which 2 frames slide on a plate h, and 

 are pressed against it by thin plates aa: the frames b and e are moved in con- 

 trary directions by turning the button d ; Lisa scale of equal parts on the frame 

 b; it is numbered from each end towards the middle with 10, 20, &c. There 

 are 2 verniers on the frame e, one at m, and the other at n, for the conveniency 

 of measuring the diameter of a planet, &c. on both sides of the zero. The 

 first division, on both these verniers, coincides at the same time with 2 zeros on 

 the scale l, and, if the frame is moved towards the right, the relative motion of 

 the 2 frames is shown on the scale l by the vernier m; but if the frame b be 

 moved towards the left, the relative motion is shown by the vernier n. 



This micrometer has a motion round the axis of vision, for the conveniency 

 of measuring the diameter of a plant, &c. in any direction, by turning an 

 endless screw f, and the inclination of the diameter measured with the horizon 

 is shown on the circle g by a vernier on the plate v. The telescope may be 



