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 oltjects, such as methyl-blue 

 stained gonococei or Klehs-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 tliree 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 



