198 THE AMERICAN MONTHLY [June 



chromatic plate does away with all these difficulties, 

 arising as they do from complex conditions of differing 

 visual and actinic focus^ of working objectives not suit- 

 able for photography, and of plates sensitive to the light 

 rays of the wrong end of the spectrum. The orthochro- 

 matic plate is sensitive to yellow light. In artificial 

 light, oil and gas light especially, yellow rays predomi- 

 nate, and when such light is used the projected image is 

 mainly formed by yellow rays, and if the image is re- 

 ceived on a plate sensitive to yellow, the visual and ac- 

 tinic focus will coincide with any objective, whether it is 

 specially corrected for photomicrography or not. Also 

 the yellow sensitive plate is so actinically sensitive to the 

 yellow light that proper molecular change is produced in 

 its silver compounds, causing in development sufficient 

 contrast with almost if not quite all stained objects and so 

 greatly shortening the exposure that it compares favora- 

 bly with those made by sunlight. For these reasons oil, gas, 

 or acetylene light, properly used in combination with 

 orthochromatic plates, gives the important necessity, an 

 always available light, and one which is at the same time 

 cheap, steady, easy to manage, and which can be used 

 with ordinary working objectives with the certainty that 

 if they give sharp visual definition they will give good 

 definition photographically. 



The remaining desideratum is an apparatus which 

 .shall be so simple and easy to manage that it can be con- 

 nected with the microscope and the projected image pho_ 

 tographed with little trouble and with a minmium 

 expenditure of time. 



The following is descriptive of an apparatus and 

 method which have been adopted by the writer after 

 .much experience in photomicrography. The means and 

 method are believed to be sufficiently simple and efi'ec- 

 tive to warrant the assumption that by them photomicro- 

 graphy may be employed for practical work. 



