The Microscope. 221 



speedily be mastered. Upon the proper lighting of the subject, 

 depends in a great measure the quality of the resultant 

 negative. 



Objects requiring to be obliquely illuminated, or by means 

 of Paraboloid, or Polariscope, may be quite as successfully pho- 

 tographed by lamp light, as those which are centrally lighted ; 

 whilst all opaque bodies capable of reflecting light are equally 

 impressionable upon the sensitized plate. In short, we have in 

 the Gelatine Dry Plate an agent, wherewith may be accurately 

 produced the image of any object seen in the field of the Micro- 

 scope, and from which Lanterns slides for projection upon the 

 screen may be made by anyone, far more accurate and truthful 

 than the most elaborate drawings of the skilled artist. 



TO BE CONTINUED. 



PROF. ROGERS' RULING MACHINE AND METHOD OF 

 RULING STANDARD MICROMETERS. 



B 



BY PROF. MARSHALL D. EWELL, M. D., LL. D., 

 UNION COLLEGE OF LAW, CHICAGO, ILL. 



Y the courtesy of Prof. W. A. Rogers, of Harvard Observa- 

 tory, I had an opportunity of spending several days with him 

 in the month of August, just ended, studying his methods of 

 ruling and comparing standard micrometers ; and as the subject 

 is one of great interest as well as importance, and as Prof. 

 Rogers is acknowledged on all hands to be the highest authority 

 in this country upon the subject of micrometry and standards 

 of length, I have thought the matter of sufficient interest to 

 warrant its publication, and have accordingly, with Prof. Rogers' 

 permission, written this brief account of the same. It will be 

 impossible for me by mere words, without drawings, to give an 

 adequate detailed description of this wonderful machine, and I 

 shall attempt only a general description of its essential features. 

 The machine consists essentially of a precision screw, 18 

 inches long, having a pitch of l-20th of an inch, with an index 

 wheel about 10 inches in diameter attached to one end, the cir- 

 cumference of which is divided on the edge into 100 equal parts. 

 This screw, by means of a half nut resting upon it, carries a 

 table upon which is placed the plate to be ruled. As you stand 



