SCREENS PLATES EXPOSURE 211 



absorption-spectrum in any particular region, glass screens are 

 not to be recommended. Recourse must then be had to some 

 of the fluid or stained film-screens made with aniline dyes, 

 or suitable chemical solutions. 



The ideal filter would absorb completely all the region of 

 the spectrum that is not required, and would transmit the 

 remainder with no loss of light. It must be admitted that no 

 known filter conforms entirely to these requirements, and even 

 among the best of them general loss of light, amounting to 

 80 or 85 %, is not excessive. 



For visual work the best known of all fluid niters is the 

 Gifford F-line screen (Fig. 67 d). This can be made by dis- 

 solving malachite-green in pure glycerin, and the strength of 

 the solution must be such that it absorbs all the red end except 

 one narrow band. Immersed in the solution is a thin piece of 

 optically worked signal-green glass, and this must not be thicker 

 than is necessary to absorb the already-mentioned narrow red 

 band ; otherwise there is too great general absorption and 

 consequent loss of light. For visual purposes this screen is not 

 to be excelled, but owing to its not having a very sharply 

 defined absorption-spectrum, it is not so generally useful 

 in photo-micrography, except when dealing with diatoms. 



The figure shows the light transmitted, and it will be seen 

 that it extends well into the violet end of the spectrum. 



In cases where achromatic lenses are being used, a series 

 of yellow filters will be found of use ; and, provided that they 

 are not capable of transmitting any of the ultra-violet part of the 

 spectrum, they will ensure a sharp image even if the objective 

 is not photographically well corrected. 



A saturated solution of picric acid is one of the best of these. 

 It has a very well-defined absorption-spectrum (Fig. 67 7c), and it 

 will be seen that the whole of the blue- violet and ultra-violet 

 is completely stopped out. It transmits the yellow and green 

 with a minimum of light-absorption. 



Of the yellow aniline dyes, the most suitable are filter- 

 yellow K, tartrazine, and aurantia (Fig. 67 g, Ji t and ?'). The 

 strength of solution of any of these, assuming that they are 

 used in solution, must depend on the thickness of the cell ; but 

 rather greater efficiency will be secured by using a very thin filter 

 say, two millimetres thick and adopting a proportionately 



p 2 



