Theory of Illuminating Apparatus. By Br. H. E. Fripp. 507 



date in the history of English invention of Microscope apparatus, 

 for since this date the claim of construction upon scientific 

 principles has been steadily preferred. It is therefore woith while 

 to look upon the theory and practice of the present day from the 

 standpoint of those who are its earlier founders, and whose mastery 

 of optical science is acknowledged. 



Sir D. Brewster states the case as it stood in 1820 with candid 

 accuracy, as follows: — 



" But the nature of the light employed, the magnitude of the 

 pencil (thrown on the illuminator), its condition with regard to 

 parallelism, divergency, or convergency, and. the diameter of the 

 pencil employed (i. e. thrown on the object), or the direction in 

 which it falls upon the object, and upon which the performance of 

 the finest instrument essentially depends, have never been discussed 

 as matters of science." 



The deduction by which Sir D. Brewster arrives at his theory 

 of illumination is based upon the following physical and phy- 

 siological grounds. If, he says, the object upon which a Micro- 

 scope is focussed be lit by a surface containing an infinite number 

 of radiant points, the images formed on the retina of the shadows 

 caused by the intervention of opaque parts of the object will not be 

 coincident, because these opaque parts cannot possibly be situate 

 exactly in the focal point of the crossed rays, and also because of 

 the spherical aberration which gives difierent conjugate foci for 

 images formed by pencils passing through the lens at difierent 

 distances from its central axis. Hence the indistinctness of the 

 total efiect produced by a number of not accurately coincident 

 (that is overlapping) images, and the importance of illuminating 

 the object from one point only. And this theory is embodied in 

 the practical directions above quoted. 



From what follows it will be seen that this physical and 

 physiological reasoning is untenable. It is sufiicient to point out 

 here that the " condenser " which Sir D. Brewster designed with 

 the special object of lighting the object from a point, upsets his 

 own theory; as the source of light is practically the surface of the 

 lens nearest to the object — a relatively large surface from which 

 pencils of considerable angular value are directed at various 

 mclination to the axis ! consequently the illumination by daylight 

 from a single hole in a window-shutter has no support from the 

 practice of artificial illumination, while it is self-evident that when 

 a mirror only is employed the illumination is simply enfeebled by 

 exclusion of light. 



It is therefore not a little remarkable that in discussing the 

 proper method of illuminating objects for the solar Microscope (by 

 "reflected " light), Sir D. Brewster discards the illumination from 

 a single point, because " in consequence of the light arriving- from 



