508 Transactions of the Society. 



one direction only' the surface of the iUnminated object is covered 

 with deep shadows, and the intensity of illumination is by no means 

 sufficient when the power of the instrument is considered." Hence 

 he proposes '• that the sun's light should be reflected by a very large 

 mirror through four apertures, each of which is furnished with an 

 illuminating lens. By these means the light would fall upon the 

 object in four different directions, and by shutting up one or more 

 of the four lenses (or parts of them) we shall be enabled to find the 

 particular direction of the light which is best suited for developing 

 the structure." 



In 1829 Dr. Wollaston's observations were made public, and a 

 drawing of his condenser given in the ' Philosophical Transac- 

 tions ' ; and we next quote from Sir D. Brewster's essay * the 

 following passage : — " The marked difference between the methods 

 of illumination proposed by Dr. WoUaston and Sir D. Brewster, 

 induced the latter to publish in 18^1 a paper " On the Principle of 

 Illumination of Microscopic Objects." In this paper the mistake 

 committed by Dr. Wollaston is clearly pointed out. The rays which 

 Dr. Wollaston throws upon the object, in place of being rays 

 actually converyed to a focus, as they ought to be, are rays which 

 diverge from a focus situated between the object and the lens. 

 He makes the focal point of the circular margin of the perforation 

 (that is, the diaphragm) fall upon the object without considering 

 that the rays which pass through that perforation do not diverge 

 from it, and therefore cannot be collected in the conjugate focus 

 corresponding to the perforation. In Dr. Wollaston's diagram t 

 the rays which are incident on the mirror are actually drawn as 

 parallel rays, and it is quite clear that he meant them to be 

 parallel rays issuing from the bull's-eye lantern which he recom- 

 mends. But if we suppose that a common flame is used, the error 

 is just of the same nature. It is a distinct image of the flame that 

 should be thrown on the object, and hence the perforation (diaphragm 

 aperture) should be placed close to the flame, the source of light 

 and the illuminated object forming the conjugate foci of the lens." 



It is not for the sake of controversy, but because the sharp 

 criticism of Dr. Wollaston's invention by Sir D. Brewster offers a 

 starting-point for a more progressive discussion of the theory of 

 illumination, that the present writer ventures in a humble way to 

 re-enact the part of sartor resartus. 



It is first of all tt) be noted that the original intention of an 

 illuminator which should collect light on the object was not to 

 condense it, but to exclude any other rays from the field of vision 

 except such as came through or from the object. Dr. Wollaston 

 lays down the following proposition which Sir D. Brewster accepts 



* 'Encvclop. Brit.,' vol. xv. p 49, 7tli cd. 

 t ' I'hif. Trans.,' 1S29, plitc ii. fig. 1. 



