ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 355 



of the top lens. Even to-day no better condenser is made for powers 

 from ^\ in. downwards. Ground-glass can be used witli or without a 

 suljstage condenser, but we are told that the orthodox method is to place 

 a piece of finely ground-glass upon the stage immediately below the slip.* 

 Ground-glass scatters the light it transmits in all directions, and therefore 

 the ol)jective will be working at full aperture. So far it would seem that, 

 with daylight and the concave mirror, all control over the working aper- 

 ture is lost ; of course the light may be reduced by the iris, but obviously 

 there is no means of varying the AV.A. to, say, f or h cone. But if the 

 unorthodox method of placing the ground-glass behind a substage con- 

 denser is adopted, we shall find, by inspecting the back of the objective, 

 that with the help of the iris we can regulate the working aperture of 

 the objective. It would seem, therefore, that this position for the ground 

 glass is a better one than that usually recommended. 



With regard to its use with higli powers the case is somewhat dif- 

 ferent ; if with a 4-ram. apochromat a ground-glass screen, placed 

 immediately behind the object, be illuminated by a substage condenser, 

 and the iris fully opened, an image not very dissimilar to that when 

 ordinary critical illumination is employed will be seen at the back of 

 the objective (No. 4) ; but when the iris is closed a marked difference 

 takes place, for the image then will not be of the ordinary form as in 

 No. 5, but will be an image of the source of light, in this case the 

 edge of the flame (No. 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 ? It is found that while ground-glass does not give the best 

 results, it simpHfies the manipulation; with medium and high , powers 

 a substage condenser should be used, otherwise the images will be poor. 



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

 are a trouble which ground-glass will get rid of, but — and this should 

 not be forgotten — at the expense of good definition. There is one more 

 point before the trial of this kind of illumination is exhausted. It may 

 be urged that while ground-glass does not give such a good image as 

 that obtained with a first-rate condenser, yet, if the condenser is a bad 

 one, it will improve the image by neutralizing some of its defects, such 

 as chromatism and errors of centricity. To determine if this were so a 

 very cheap condenser, consisting of two single plano-convex lenses, was 

 tried with an achromatic J in., with and without ground-glass, and the 

 image was found to be better without the ground-glass. 



Screens^ — Formerly we all made a mistake by pitching our illumina- 

 tion too high up in the spectrum (I am now speaking of visual, not of 

 photographic work). There can be no doubt about this, because fine 

 detail is lost if the light is too high up in the spectrum. The cause is 

 probably a physiological one. Experiments show that a normal eye is 

 more sensitive in picking up fine detail when the light is a peacock- 

 green. Although with light high up in the spectrum the resolving 

 power of a lens is increased, yet the sensibility of the eye is diminished. 



* Invented by John Keates, of Liverpool. 



t Screens were first introduced by Sir D. Brewster in 1836. His screen was 

 a red one. 



