jackson's eye-piece micrometer. 05 



Tube has been drawn out, as marked by the graduated scale of 

 inches which it should possess. And he should also make an 

 accurate estimate of the thickness of the Cobweb-threads them- 

 selves ; since, if this be not properly allowed for, a serious error 

 will be introduced into the measurements made by this instru- 

 ment, especially when the spaces measured are extremely minute. 

 (See Mitchell in "Transact. Micros. Soc." Vol. xiv. p. 71.) 



68. The costliness of the Cobweb Micrometer being an important 

 obstacle to its general use, a simpler method is more commonly 

 adopted, which consists in the insertion of a transparent scale 

 into the focus of the Eye-piece, on which the image of the object 

 is seen to be projected. By Mr. Andrew Ross, who first devised 

 this method, the 'positive' Eye-piece (§ 22) was employed, and a 

 glass plate ruled in squares was attached beneath its field-glass, at 

 such a distance that it and the image of the object should be in 

 focus together ; and the value of these squares having been deter- 

 mined with each of the Objectives, in the manner already described, 

 the size of the object was estimated by the proportion of the 

 square that might be occupied by its image. While the use of the 

 positive eye-piece, however, renders the definition of the ruled 

 lines peculiarly distinct, it impairs the definition of the object ; 

 and the ' negative' or common Huyghenian Eye-piece is now gene- 

 rally preferred. The arrangement devised by Mr. Gr. Jackson, 

 allows the divided glass to be introduced into the ordinary Eye- 

 piece (thus dispensing with the necessity for one specially adapted 

 for micrometry), and greatly increases the facility and accuracy 

 with which the eye-piece scale may be used. This Scale is ruled 

 like that of an ordinary measure (i.e.. with every tenth line long, 

 and every fifth line half its length) on a slip of glass, which is so 

 fitted into a brass frame (Fig. 51, b) as to have a slight motion 

 towards either end ; one of its extremities is pressed upon by a 

 small fine milled-head screw which works through the frame, and 

 the other by a spring (concealed in the figure) which antagonizes 

 the screw. The scale thus mounted is introduced through a pair 

 of slits in the Eye-piece tube, immediately above the diaphragm 

 (Fig. 51, a), so as to occupy the centre of the field ; and it is 

 brought accurately into focus by unscrewing the glass nearest to 

 the eye, until the lines of the scale are clearly seen. The value 

 of the divisions of this scale must be determined by means of a 

 ruled Stage-micrometer, as in the former instance, for each Objec- 

 tive employed in micrometry (the drawing out of the eye-piece 

 tube enabling the proportions to be adjusted to even and convenient 

 numbers); and this having been accomplished, the Scale is brought 

 to bear upon the object to be measured, by moving the latter as 

 nearly as possible into the centre of the field, and then rotating 

 the Eye-piece in such a manner that the scale may lie across that 

 diameter which it is desired to measure. The pushing-screw at 

 the extremity of the scale being then turned until one edge of the 



