and Laboratory Methods. 



•2083 



Before exposing the plate a delay of a few minutes is necessary to see that 

 the focus does not slip. We have been able to overcome, in a measure, the 

 annoyance of a changing focus by discarding the spring clips that hold the slide 

 to the stage of the microscope. Either by doing this or by using a mechanical 

 stage, one of the factors of a changing focus is removed. 



As the scale of the stage micrometer cannot be seen distinctly enough on the 

 ground glass with daylight illumination to admit of accurate measurement, we 

 have heretofore ascertained the magnifications of our photographs by measuring 

 the subject with a Zeiss micrometer eyepiece (using '2 mm. objective), taking 

 the measurement of the photograph in microns, and dividing the latter by the 

 former. We have found that this method does not give accurate results, because 

 the Zeiss micrometer eyepiece, as tested by the Zeiss stage micrometer, over- 

 estimates the size of the subject, thus causing the magnification of the photograph 

 to be underrated. This can be seen by measuring the Zeiss stage micrometer by 

 the Zeiss micrometer eyepiece. With the Zeiss 2 mm. objective and a tube 



Zeiss stage micrometer ruled to 0.01 mm. spaces. In photo each space measures 10 mm. 

 showing a magnification of 1000 diameters. 



length of 153 mm. (with nosepiece), 90// of the stage micrometer measures 

 with micrometer eyepiece 97//, and it is necessary to shorten the tube to the 140 

 mark to make the two scales agree. 



A few photographs of the stage micrometer, taken with different combinations 

 of lenses and bellows draw, provide an accurate register of magnifications, in 

 convenient form for reference in selecting the lenses and draws needed for a 

 given magnification. 1 



Fig. 1 is a reproduction of a photograph of a part of a Zeiss stage micrometer 

 taken with a Zeiss 2 mm. apo. immer. lens, 140 aperture, Zeiss projection ocular 

 4, diaphragm at 0. tube length 153 mm. (with nosepiece) and bellows draw of 27 

 inches from stage of microscope to plate. The focus was taken on the faint dot 

 in the center of line 6. This dot was placed exactly in the center of the held 

 with the aid of the Zeiss micrometer eyepiece. We have reproduced this photo- 



' With the Zeiss 2 mm. immer. lens, projection ocular 4, diaphragm at 0, we have always used a 

 29 inch bellows draw for about 1000 magnification. This was given in an earlier paper as 293^ 

 inches, the error arising from the measurement being taken to top of the upper collar, instead of 

 from stage of microscope to plate, as stated. Testing this measurement with a photograph of a 

 stage micrometer shows that 27 inches is the correct draw for a magnification of 1000. 



