78 THE USE OF THE MICROSCOPE 



among others, now make special yellow-green glasses, 

 which may perhaps be used in place of some of the Wratten 

 screens. 



Adjusting Intensity of Light. — The degree of intensity 

 of the light may be altered to suit the combinations of 

 objectives and eyepieces, by the use of screens or otherwise: 



1. By using a series of yellow-green screens of increasing 

 density as already described. The five screens here men- 

 tioned, used singly, are stated by the makers to transmit 

 the following percentages of the incident light: No. 66, 

 58 per cent; No. 56, 48 per cent; No. 57A, 34 per cent; 

 No. 58, 23 per cent; and No. 61, 18 per cent. That is, 

 Wratten screen No. 66 passes nearly three-fifths of the 

 light; No. 56, nearly one-half; No. 57A, one-third; No. 58, 

 nearly one-quarter; and No. 61, nearly one-fifth. 



2. By using a neutral-tinted wedge of glass, or gelatin 

 enclosed in glass, together with a compensating wedge; 

 a method which gives accurate gradations. 



3. By using a series of neutral screens, either of tinted 

 gelatin (Wratten), or of platinized glass (Barnard). 



4. By a variable resistance on the electric lamp (Denham, 

 58). 



5. By superposing pieces of ground glass (Beck). 



Thus, if artificial daylight is used, one of the Wratten 

 series of neutral gelatin films, or a set of similar screens, is 

 required for maximum working aperture and maximum 

 useful magnification. 



An excellent investigation of the transmission for different 

 colors is to be found in the pamphlet on the Wratten light 

 filters (62). 



Summary. — Yellow-green light filters rest the eyes, help 

 much to correct objective and condenser, give greater 

 contrast and better definition, allow regulation of light, 

 cut out much or all red, and so decrease the wave-length 

 of the brighter part of the light for vision ; and also increase 

 the acuity of vision in the eye itself. If yellow-green 

 filters are not employed, Wratten daylight film, or the 

 excellent Corning daylight glass, may be used, along with 



