[Annals N. V. Acad. Sci., XI, No. i8, pp. 405 10406, October ij, 1S98.] 



photographp:d ocular micrometers. 



Wallace (iooLD Levison. 



(Read April 4. 1898.) 



Owing probably to the difficulty of starting and stoppinir rul- 

 ing machines at cross lines without overrunning them, it has 

 been found difficult to obtain eye piece micrometers ruled in 

 squares, particularly of the design now so much used in water 

 supply investigations for counting and measuring micro-organ- 

 isms. 



It occurred to me that these micrometers might be made 

 easily by photography, and as a test experiment I made some 

 of them by the ordinary simple dry-plate method with some 

 precautions to ensure clear films. An outline drawing 14 centi- 

 meters square was first made with India ink and an ordinary 

 drawing pen upon glass coated with gelatin, and one-half the 

 square in each direction was ruled in fiv^e equal parts. The 

 small central square formed by the crossing of these lines was 

 then divided by cross lines into twent}^-five equal areas accord- 

 ing to the plan given in Prof. Albert R. Leed's report on the 

 Brooklyn water.^ 



This drawing was photographed down b}' an ordinary one- 

 quarter portrait lens with small diaphragm .stop, to about five 

 centimeters square on a Stanley dry plate, care being taken to 

 obtain as nearly as possible a black negative with very clear 

 lines. For each micrometer this is again reduced by the same 

 lens to a square of seven millimeters on lantern slide plate, care 

 being taken to develop the lines black/ keep the film transparent 

 and avoid scratches. 



^ A. R. Leeds, Report on the Brooklyn Water, published by the Department of 

 City Works of Brooklyn, X. Y., 1897. 



ANNAL.S X. Y. Acad. Scr., XL, October 13, 1898—27. 



( 405 ) 



