294 E. M. NELSON ON METHODS OF ILLUMINATION. 



image of the source of light, in this case the edge of the flamer 

 (Fig. 6). This illumination is asymmetrical with regard to the 

 aperture of the objective and therefore should be avoided. Now,, 

 what is the effect of the ground glass ; does it improve or spoil 

 the image i After exhaustive trials with lenses of various powers 

 and apertures upon different objects the results may be summed 

 up by one word " Fog." With ground glass the clear parts of 

 the field are not so bright, and the dark parts are softer in tone 

 than without it; in fine, the image, to use a photographic term, 

 lacks the " pluck" of a critical picture, as if a thin veil had been 

 spread over it. Ground glass gets rid of all necessity of centring 

 the illuminating beam, for it is only necessary to place the mirror 

 in such a way that light falls upon the ground glass anyhow.. 

 If the Ramsden disc be examined it will be found full of diffused 

 light ; further, the mirror may be moved about in various direc- 

 tions and, providing of course that the light is not turned 

 altogether away fi-om the ground glass, no change will be 

 observed in the image, but if no ground glass is used the slightest 

 displacement of the miri'or from its proper position will decentre 

 the image and mar the definition. While ground glass does not 

 give the best results, it simplifies the manipulation ; with medium 

 and high powers a substage condenser should be used, otherwise 

 the images will be poor. As this was being written, an achromatic- 

 5 in. of 0*7 7 N.A., with a coarse Navicula lyra, concave mirror 

 (2| in. in diam.) and daylight, without any condenser, was tried 

 with and without ground glass. Poor as both images were, that 

 without the ground glass was the better. The field leus (about 

 2 in. focus) of a B eyepiece was then placed below the stage, as 

 a condenser, without ground glass, and it was found that the- 

 image was improved. If, then, the miserably poor image obtained 

 from such an elementary form of condenser is better than that 

 seen without it, we can understand how very bad the image with 

 the ground glass must have been. 



With low powers, window bars, moving clouds, chimney-pots,, 

 etc., are a trouble which ground glass will get rid of, but and 



