250 PROCEEDINGS OP THE AMERICAN ACADEMY 



that it is the result of polishing in one direction. Common window 

 glass seems to be wholly free from it. Nobert's lines are ruled on 

 microscopic cover-glass about ^l^ of an inch in thickness. The evi- 

 dence of grain in this kind of glass is strong ; but it is hardly decisive. 

 In some specimens it is very marked, while in others it seems to be 

 entirely wanting. Indeed, any conclusions on this subject must be 

 regarded as only provisional, owing to the extreme difficulty of sep- 

 arating the action of the cutting crystal upon the glass from the effect 

 due to the character of the glass itself. It is, however, safe to say 

 that in certain kinds of glass the best results can only be obtained by 

 ruling in a given direction. 



In order to rule bands with lines separated by intervals, e.g. of 

 ^0000 ^^^ inch, it is of course necessary to rule single lines whose width 

 is less tliau this. Great precaution is requisite here, in order to avoid 

 optical delusion. Every microscopist is familiar with the phenom- 

 enon of false lines. To avoid errors from this source, a few single lines 

 are ruled between two heavy finding lines. They are then filled with 

 graphite. This precaution is necessary in order to give both visibility 

 and distinctness to the edges. If the lines are not filled, they may 

 appear much finer than they really are ; that is, the objective being 

 in focus for the bottom of the furrow may fail to reveal abrasions of 

 the surface on either side. The graphite of the New York Graphite 

 Company will easily fill the finest line that can be ruled with a 

 diamond. 



In order to measure the width of the lines, the following plan is 

 adopted as presenting some advantages over the usual method of esti- 

 mating it by comparison with the known value of a given division in 

 the eye-piece micrometer. 



First, a single line is ruled, which in the eye-piece apparently ex- 

 actly covers the line to be measured under the objective. A few trials 

 will suffice for this purpose. Having found what weight must be 

 applied to the diamond to produce such a line, the next step is to 

 ascertain how many lines, exactly like this one, can be ruled within 

 the space of ^^-j^ of an inch with a minimum space between each line. 

 This will also require a few trials. For example, if with a -jV objec- 

 tive and a B ocular, the space ti^tttt ^^ ^^ '"^^ ^^ the eye-piece corre- 

 sponds to jshjj^ of an inch under the objective, and if it is found that 

 fifteen lines can be ruled within this space, then the width of the line 

 under examination is tj^tttt X tV = ^^^^rrtTTT ^^ f*" '"^'^ 5 ^ result 

 which is obviously within the truth, especially if the line in the eye-piece 

 is made a shade larger than the line under the objective. Tested in 



