52 DAYLIGHT GLASS [Cn. II 



placed opposite the filament of the lamp as shown in fig. 37, where 

 m c represents the filament. 



So used, the brightly illuminated frosted daylight glass becomes 

 practically the source of light for the microscope, and resembles very 

 closely that from a white cloud. There is no glare, the color values 

 are correct, and if a loo-watt, nitrogen-filled, mazda lamp is used, the 

 light is abundant for all powers of the microscope up to and in- 

 cluding the 1.5 mm. oil immersion. 



For objectives up to 8 mm. it is best to have the daylight glass 

 ground on both sides. The diffusion will then be sufficient to 

 give a uniformly lighted field. For powers of 4, 3, 2, and 1.5 

 mm, focus it is better to have one side of the daylight glass 

 polished and one side ground. This gives sufficient diffusion of 

 the light from the source to fully and evenly light the field, and 

 as the diffusion is less the brilliancy of the light will be corre- 

 spondingly greater from the smaller area. A good plan is to have 

 one opening of the lantern (fig. 37-38) with a disc of glass ground 

 on both sides for low power work and another with the daylight 

 glass frosted on one side and polished on the other for high powers. 



It -may be said that the position of the ground-glass filter should 

 be close to the source of light. Its brilliancy will vary inversely with 

 the square of its distance from the lamp filament. If the daylight 

 glass were polished, then it could be used anywhere between the source 

 of light and the eye; for example, under the condenser in the usual 

 place for polished colored glasses, or over the ocular. The advantage 

 of having it ground and near the lamp filament is that one can get a 

 uniform light and wholly avoid the image of the lamp filament. Fur- 

 thermore, when using a lamp it should be enclosed to avoid the general 

 flooding of the room with unfiltered light, to say nothing of the annoy- 

 ance to the observer and his coworkers. The enclosure in a lantern 

 (fig. 37-38) avoids all that 



92a. For a discussion of the requirements for the production of artificial 

 daylight, and the means so far employed, and the uses of artificial daylight, see: 



Herbert E. Ives. Artificial Daylight. Journal of the Franklin Institute, 

 vol. 177, May, 1914, pp. 471-499. 19 figures. 



Simon H. Gage. Artificial Daylight for the Microscope. Science, N. S. f 

 vol. 42, October, 1915, pp. 534-536. One curve. 



