218 PRACTICAL PHOTO-MICEOGRAPHY 



considerable general loss of light is unavoidable. It is possible 

 to improvise liquid filters which have somewhat less general 

 absorption, although they may not so efficiently cut out that 

 portion of the light that is not required. The series, as sold 

 by Messrs. Wratten & Wainwright, consist of seven niters, 

 and with each one particulars are -given of the wave-length it 

 transmits. 1 



Messrs. Wratten & Wainwright also issue another series for 

 use with a mercury- vapour lamp. The most important filter 

 of this series is made of didymium glass from the Schott 

 glass-works in Jena. It is somewhat expensive, but it has 

 the highest efficiency of any that are at present known for 

 isolating the green line of mercury vapour. 



Sensitive Plates. There are at present practically only two 

 types of plate that are available for the ordinary worker : 

 gelatin dry-plates, or wet collodion. The former are obtainable 

 .commercially ready for use, but the latter have to be coated 

 as required. Wet collodion was formerly the only available 

 process, and it must be admitted that in the hands of experts 

 it gave results that have never been excelled. But the 

 inconvenience of having to coat a plate before each exposure 

 gives an advantage to the gelatin dry plate that to the 

 ordinary worker is irresistible. 



In photo-micrographic work the requirements of a plate 

 are that it should give a bright image with as fine a grain as 

 possible ; that is, that however fine the detail in the plate, 

 the grain of the silver particles in the emulsion should not be 

 sufficiently large to become evident. In this respect collodion 

 has very much the advantage, as the grain of a collodion plate 

 is much finer than any gelatin one ; in fact it partakes 

 more of the nature of a stain than of a definite deposit 

 in the film. 



A compromise is possible by using a dry collodion emulsion, 

 and such an emulsion ready prepared is now to be obtained 

 commercially, leaving only the coating of the plates to be 



1 An Atlas of absorption-spectra, by Dr. C. E. Kenneth Mees, has recently 

 been published by Messrs. Wratten & Wainwright. In it the absorption- 

 spectra of nearly all dyes used in staining microscopic preparations are given, 

 together with photographs of the spectra transmitted by a large series of 

 screens that they now make. This work is the most complete on the subject 

 that has yet been issued, and the reader is referred to it if further exact 

 information on the subject is desired. 



