Theory of Illuminating Apparatus. Bij Dr. H. E. Fripp. 509 



as the same, " thougli not so fully developed," as his own proposi- 

 tions enunciated in 1 and 5. " In the illumination of microscopic 

 objects, wliatever light is collected and brought to the eye beyond 

 that which is fully commanded by the object-glasses tends rather to 

 impede than to assist vision." In the next place. Dr. Wollaston 

 plainly states that the intensity of illumination will depend upon 

 the diameter of the illuminating lens and the proportion of the 

 image of the aperture to its actual size, " which may be regulated 

 according to the wish of the observer." Here we see the " marked 

 difference between the principles respectively advocated by Dr. 

 Wollaston and Sir D. Brewster. The former advocates the 

 principle of illumination from a large radiant surface, employing at 

 the same time a method of regulating the amount of hght derived 

 from the original source by a diaphragm perforation. The latter 

 arranges a chcumscribed and small area of light-source, a point of 

 light which is transferred to the object as a point of hght of exactly 

 the same size, the two points being conjugate foci. Now it is 

 obvious that in both illuminators the intensity of light is not 

 gained by condensation, but by nearer approach to the object ; the 

 brightness being inversely proportional to the square of the distance 

 from the light-source. It is equally certain that in both illuminators 

 the surface of the lens nearest the object is ptractically the light- 

 giving source.* In the illuminator of Sir D. Brewster the light 

 spreads first upon the collecting surface of a lens much larger than 

 the diaphragm aperture, and after various changes of direction is 

 again concentrated on the object. Yet in reforming the original 

 point of light the rays previously dispersed until they occupy the 

 whole surface of the lens which focusses them on the object, fall in 

 from this radiant area with converging lines of various inclination 

 to the axis : thus fulfilling the very conditions which Sir D. 

 Brewster considered unsuitable, and which he sought to remove by 

 an apparatus which, after all, acts upon the " mistaken principle " 

 charged upon Dr. Wollaston ! Fortunately, however, for micro- 

 scopy, the modern objective not only enables, but demands, the 

 adoption of this principle, which perhaps in 1829 Sir D. Brewster 

 had some grounds for denouncing as a mistake when single lenses 

 with great spherical aberration were the only objectives in use. 



It remains now to examine Sir D. Brewster's criticism, as quoted 

 above, on Dr. Wollaston's optical diagram and its explanation. 

 The rays incident on the mirror are drawn parallel (the same 

 blunder is repeated to-day in our handbooks !), and Sir D. Brewster 

 says truly that the image of the diaphragm aperture could not fall 



* Anyone may satisfy himself of this fact by looking at a light (of any kind) 

 through a lens placed before the eye at the principal focal distance ; when the 

 whole surface of the lens will be seen equally radiant, while the primary source 

 is not seen. 



VOL. II. 2 M 



