ACCESSORY APPARATUS. 



93 



to touch the stationary filament, the other thread is moved by the micro- 

 meter-screw until it appears to lie in contact with the other edge of the 

 object ; the number of the entire divisions on the scale shows how many 

 complete turns of the screw must have been made in thus separating the 

 filaments, while the number to which the index points on the milled- 

 head shows what fraction of a turn may have been made in addition. It 

 is usual, by employing a screw of 100 threads to the inch, to give to 

 each division of the scale the value of l-100th of an inch, and to divide 

 the milled -head into 100 parts ; but the absolute value of the divisions is 

 of little consequence, since their micrometric value depends upon the 

 Objective with which the instrument may be employed. This must be 

 determined by means of a ruled slip of glass laid upon the stage ; and as 

 the distance of the divisions even in the best-ruled slips is by no means 

 uniform, it is advisable to take an avarage of several measurements, both 

 upon different slips, and upon 

 different parts of the same slip. 

 Here the Drawtube will be of es- 

 sential use, in enabling the Micro- 

 scopist to bring the value of the 

 divisions of his Micrometer to 

 even numbers. The Microscopist 

 who applies himself to researches 

 requiring micrometric measure- 

 ment, should determine the value 

 of his Micrometer with each of 

 the Objectives he is likely to use 

 for the purpose ; and should keep 

 a table of these determinations, 

 recording in each case the extent 

 to which the 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 thick- 

 ness of the Cobweb-threads them- 

 selves ; since, if this be not prop- 



i 11 a j 1 Spectroscopic appearance of fresn focariet 



erly allowed for, a SeriOUS error Blood- 2 of Deoxydized Blood (cruorine) ; 3, of 



will be introduced into the mea-ggna^^^^^byj^^ncraoriiie wiA 

 surements made by this instru- ac 



ment, especially when the spaces measured are extremely minute. (See 

 Michell, in "Transact. Micros. Soc." K". S., Vol. xiv., p. 71.) 



91. The costliness of the Cobweb Micrometer being an important 

 obstacle to its general use, a simpler method (devised by Mr. G-. Jackson) 

 is more commonly adopted; which consists in the insertion of a transpar- 

 ent scale into an ordinary Huyghenian Eye-yiece in the focus of the eye- 

 glass, so that the image of the object is seen to be projected upon it. 

 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. 66, B), as to have a slight motion towards 

 either end; one of its extremities is pressed-upon by a fine milled-head. 

 screw which works through the frame, and the other by a spring (con- 

 cealed in the figure) with antagonizes the screw. The scale thus mounte 

 is introduced through a pair of slits in the Eye-piece tube, immediately 

 above the diaphragm (Fig. 66, A), so as to occupy the centre of the neld; 



