110 THE MICROSCOPE AND ITS REVELATIONS. 



Although specially designed as a ' black-ground ' illuminator, it may also 

 be made useful in the resolution of difficult Test-objects by transmitted 

 light, 1 the illuminator being lowered until a colored spectrum appears in 

 the field, the rays of which bring out their markings with remarkable 

 distinctness. For use with either of these arrangements for 'black- 

 ground ' illumination, it is better that the objects should be mounted 

 ' dry/ especially when they are to be viewed under 'immersion' objec- 

 tives; balsam-mounted objects being thus seen better with dry-front 

 objectives. 



108. The following directions are given by Mr. Schulze ("English 

 Mechanic," 1877, No. 661) for the use of two illuminators last described: 

 "First, rack up the Sub-stage, until the plane top of the illuminator 

 is level with the stage; centre carefully; put a drop or two of glycerine 

 on the under side of the slide, taking care that no air-bells are formed; 

 and place the slide on the stage. If, now, rays parallel to the optic axis 

 are thrown up by the plane mirror or rectangular prism, a luminous spot 

 will appear on the slide if an object lies in the optic axis. Next focus; 

 and by adjusting the mirror or rectangular prism more carefully, the 

 object will be brilliantly illuminated by very oblique rays on a black 

 ground. ... I generally use one of How's common Microscope lamps 

 filled with good paraffin oil, and having a wick half an inch broad; but 

 for the highest powers I have recourse to the Dallinger lamp ( 131). 

 After I have obtained the best results, I interpolate a bull's-eye Con- 

 denser to increase the light, focussing carefully a miniature image of the 

 flame on the slide. I invariably use the narrow side of the flame turned 

 towards the mirror or prism, when resolving lined tests. It is, however, 

 by sunlight that the performances of the Immersion Paraboloid and 

 Keflex Illuminator seem to eclipse any resolution that can be obtained by 

 transmitted light." [This was written before Mr. Schulze had found 

 out the mode of working these instruments already noticed.] In regard 

 to the relative values of the two illuminators, Mr. Schulze states as the 

 result of careful comparative trials of them: " The Paraboloid is a trifle 

 easier managed, gives a little more light by lamplight, and is somewhat 

 cheaper than the Keflex Illuminator. Both perform equally well on dark 

 ground by sunlight; but the Reflex Illuminator can also be used on 

 balsamed slides and-with immersion lenses for the examination of objects 

 by transmitted very oblique white light." 



109. Light-Modifiers. For (1) reducing the intensity either of Solar- 

 light or Lamp-light, (2) for correcting the yellowness of the latter, and 

 (3) for the equable diffusion of either light over a large field, it is often 

 convenient to employ interposed media, the nature of which must be 

 varied according to the particular purpose to be attained. The direct 

 rays of the Sun are very little employed by Microscopists, except for 

 Ph6tography or some other special purpose. But when recourse is had 

 to them in ordinary Microscopy, it is well to take advantage of ' Ramey's 

 Light-modifier/ which is a combination of one thickness of dark-blue 

 glass free from any tint of red, another of very pale blue with a slight 

 shade of green, and two of thick white plate-glass, all cemented together 

 by Canada balsam. This is mounted by Messrs. Powell and Lealand on 

 a separate stand; and may be used with Lamp-light as with sunlight. 

 Some observers use Lamp-chimneys of either neutral-tint or bluish glass 



'See Schulze in "Journ. Roy. Microsc. Soc.," Vol. i. (1878), p. 45: and Col. 

 Dr. Woodward in same Vol., p. 248. 



