406 LEVIS ON. 



The plates thus obtained are cut in circles a little larger than 

 the recess in the eye piece diaphragm in which they are to be 

 used. A cover class is then applied with balsam and xylol and 

 baked for several days until the cement is hard and dr}\ The 

 circle is finally accurately centered on a lathe and ground to a 

 true circle of the exact size of the recess in the eye piece dia- 

 phragm. 



In making these photographs the action of halation will cause 

 the lines to be much thinner in the negative than in the original 

 drawing and thicker in the finished positive than in the nega- 

 tive, and moreover it will cause a peculiar thickening where the 

 lines intersect unless the precautions known to expert photog- 

 raphers are employed at each step to counteract this peculiarity 

 of the photographic process. 



But even if not wholly obviated, this does not materially in- 

 terfere with the practical utility of the micrometer. I have no 

 doubt that very accurate and beautiful micrometers may be thus 

 made by the so-called process method which is a wet plate 

 method used for making photo engravings as it affords jet black 

 lines on a particularly clear ground. The lines as I have made 

 them are thicker perhaps than is necessary, but this does not 

 appear to interfere with the use of the micrometer, providing 

 distances are taken from one side of a line to the same side of 

 the next line, and so on throughout the scale. 



Eye piece micrometers made by the simple method I have 

 tried appear to be satisfactory for use with any objective, as re- 

 gards transparency. In fact they seem in some respects to be 

 more satisfactory than ruled micrometers, especially in the cir- 

 cumstance that the lines are black and always distinctly visible 

 and that they can be made with facility of any design desired. 

 For the latter reason they may be valuable not only for meas- 

 uring micro-organisms but also any class of microscopic objects 

 whatever, as for example the areas of the crystals or grains of 

 minerals in thin sections of rocks and building stones and 

 thereby perhaps estimating their relative proportions. 



