220 PEACTICAL PHOTO-MICEOGKAPHY 



Of the suitable isochromatic plates the following may be 

 taken as typical : 



Barnet orthochromatic plate; 



Edward's isochromatic, medium rapidity; 



Ilford orthochromatic plates ; 



The Isolar plate ; 



Lumiere, both red and yellow, sensitive plates. 



Wellington's orthochromatic process plate ; 



Wratten & Wainwright's Panchromatic plates. 



The above are merely examples of what may be regarded 

 as suitable, and there are doubtless others which will fulfil the 

 purpose equally well. The plate that the writer has found as 

 good as any is the Wellington orthochromatic process plate ; 

 it is slow, its orthochromatic properties are all that may be 

 required, and on development the grain is exceedingly fine. 



Whichever plate is chosen, it is important that it should be 

 backed that is, on the side of the glass other than that on 

 which the emulsion is coated, a layer of some black material is 

 applied to ensure that there is no reflection from the back 

 surface of the glass. On any unbacked plates, where there 

 are violent contrasts of light and shade, the effect known as 

 ' halation ' is liable to occur. This consists essentially of a 

 spreading action of the light beyond the margin of what is 

 really the properly defined image, and results therefore in a 

 certain indefiniteness at what would otherwise be an edge of 

 perfectly sharp definition. It arises from the light which 

 passes through the emulsion being reflected from the back 

 surface of the plate. This light is scattered over the back of the 

 emulsion, resulting in some photographic action where it is 

 not wanted. If the reflection of the light from the back of the 

 plate is prevented, the spreading action is very much minimised. 

 Plates may be commercially procured ready backed, so that 

 it entails no extra work except that the backing generally has 

 to be removed either before development or at an early stage in 

 the process. 



In photographing objects with very fine detail in them, 

 unless the plate is efficiently backed, there is considerable loss of 

 detail; and this, in some cases where the contrasts are very 

 great, makes it almost impossible to obtain certain results 

 except on backed plates. 



