204 THE AMERICAN MONTHLY [June 



eters with exposures of from one and one-half to three 

 minutes. Oil and gas lights are themselves so yellow 

 that with them light filters are only required when pho- 

 tographing very difficult objects, such as methyl-blue 

 stained gonococci or Klebs-Loeffler bacilli (Fig. 6). 

 When a light filter is used, a light yellow one of an 

 aqueous solution of bichromate of potash placed in a 

 glass trough gives excellent results. With it, exposure 

 is somewhat lengthened, being from three to five min- 

 utes for amplifications of one thousand diameters. 



With acetylene light a light filter is more frequently 

 required. This is due to the greater whiteness of the 

 light and its consequent effect when transmitted through 

 actinic-colored objects. With it most stained sections of 

 tissue photograph well without a filter, the exposure re- 

 quired being very short, usually varying from five to 

 thirty seconds. When a light filter is used the exposure 

 is lengthened, but is short compared with that required 

 with oil or gas light, being about two minutes for ampli- 

 fications of one thousand diameters (Fig. 3). A good fil- 

 ter for acetylene light is made by dissolving ten grams of 

 potassium bichromate in two hundred cubic centimetres 

 of water and using at a thickness of three centimeters in 

 a parallel-sided glass trough. 



Adjustment of the Apparatus. — The camera being 

 hung on the rackwork^ the microscope is placed beneath 

 it and the lantern is fixed about twelve inches in front 

 of the microscope, with its central long axis in a plane 

 which extends through the centre of the microscope mir- 

 ror, the substage condenser, the objective, ocular, and 

 centre of camera. 



The light (oil, gas, or acetylene) being lighted and 

 placed in the lantern, a stage micrometer is placed on the 

 microscope stage and a medium-power objective and eye- 

 piece are attached to the microscope. Light from the 

 lantern is reflected on the micrometer by the mirror of 



