Diazotype or Primuline Process
each diaphragm requiring double the exposure required by the preceding diaphragm.
Should the greatest effective aperture of a lens not conform exactly to an intensity set forth above, such aperture should be marked in accordance with the definition of true effective aper-ture, but all succeeding smaller apertures should be marked in uniformity with the intensities recommended in the above sequence.
Those who are specially interested in the most extreme refine-ments in the matter of lens aperture should study three recent articles by Mr. Welborne Piper, in which he treats of the Abbe Theory of Lenses (The Amateur Photographer, 2nd vol. for 1901, pp. 292, 312, and 333). Still, if we put the highest and most recondite refinements aside, and look at matters from a practical standpoint, the method of determining the effective aperture mentioned above as being put forward on the authority of the Royal Photographic Society, is satisfactory. The diameter of the beam as it emerges from the front lens may be measured with the ordinary dividers if the front surface of the lens is puffed over with starch powder, and a photographic method is to support a piece of bromide paper against the front face of the lens, and make an exposure.
Diazotype or Primuline Process. This is a process patented in 1891 by Messrs. Green, Cross, and Bevan, and is based upon the property which certain diazotised dyes possess of being so altered by light that they will not form colouring matters with certain anilines and phenols. As various colours can be obtained, and the process is applicable to paper material or gelatine on glass, it affords a ready means of obtaining decorative subjects, although the colours are not very brilliant, nor is the ground pure white. The solution of primuline is prepared as follows : -
Primuline ... ... ... ... ... 10 parts.
Distilled water ...... ... ... 1,000 ,,
The water should be heated to near boiling-point, and kept at this temperature by means of a spirit lamp or Bunsen burner, and the primuline added, with constant stirring till dissolved. In this hot solution the linen, silk, plush, or velvet - the two former being deprived of their dressing by washing - should be immersed, and care taken that the dye is evenly distributed through the material