THE president's ADDRESS. 253 



condenser has been apochromatized by Mr. Powell by tbe 

 employment of fluorite lenses. The result of this is that objec- 

 tives can be illuminated by larger solid axial cones than was 

 possible before. As the Abbe condensers have been discussed 

 above we will pass on to a somewhat later development of 

 microscopical technique, viz., monochromatic illumination and 

 screens. Monochromatic illumination for the microscope, 

 though a very old idea, being first suggested by Sir D. Brewster 

 (circa 1836), has lately come into special prominence. He pro- 

 posed three methods — first, a monochromatic lamp ; secondly, 

 absorption media, both by fluids as well as by glasses ; thirdly, 

 a prism. The intention at that date was to obviate the 

 necessity for achromatism in the objective. The objectives he 

 used were his own grooved sphere, and Herschel's doublet of no 

 aberration. To what a length this notion was pushed the 

 following passage will show : — " A solar telescope should never 

 be an achromatic one, but should consist of a compound lens of 

 no aberration, all the colours of the spectrum but one being 

 absorbed by the darkening glass." It is also curious to note 

 that a monochromatic red band was selected as being the most 

 suitable. 



Dr. Goring (1837) severely criticises Brewster's article. He 

 says that monochromatic light, even wdien obtained by means 

 of a prism, permits dispersion phenomena to be observed when 

 the light is oblique for the resolution of lined tests ; he also 

 says that he prefers blue light. His term monochromatic is 

 not used in so strict a sense as we should use it. He evidently 

 means a broad band of approximately one colour ; this would 

 fully account for the dispersion phenomena he observed. But 

 at the present time the intentions underlying the use of 

 monochromatic light are quite diiferent to those quoted above. 

 They are five in number. First, the increase in the 

 elective aperture of the objective by the shortening of 

 the wave length ; secondly, the sharpening up of the 

 image by the removal of the spherical aberration of the 

 chromatic difi'erence ; thirdly, the rendering of colours that 

 would prove refractory in photomicrographic work neutral ; 

 fourthly, the rendering of the visual and actinic foci identical ; 

 fifthly, the subduing of the intensity of the light for visual 

 purposes wlien large cones of illumination are employed. 



