512 Transactions of the Society. 



view of tlic matter, tries to explain it as an effect produced by the 

 object upon tlie course of the rays passing into the objective, and 

 proposes an eclectic mode of solving the difficulty, by employing 

 parallel, converging or diverging rays, according to the nature 

 of the object, and other cu-cumstances special to each case. He 

 further says, speaking of the means by which, as he supposed, a 

 proper direction could be given to the illuminating rays, that the 

 slia^e of the plane mirror was a matter of indifference, while of 

 course the concave mirror must necessarily be round. Goring, on 

 the contrary, maintained that the mirror should be shaped ellipti- 

 cally, and as large as possible (5 x 4). Schleiden remarks that 

 where it is possible, light from a plane mirror is to be preferred, 

 because parallel rays are more advantageous for correct observa- 

 tion, although they give a less intense hght. " For it ajjpears," he 

 adds, "as though a shifting of the image was occasioned by the 

 convergence of rays reflected from the concave mirror." But 

 though he has often observed this, he confesses that he cannot 

 explain it, and that " our opticians leave us in respect to this matter 

 quite in the dark." * 



Dr. Carpenter, in his article on the Microscope,! follows Sir D. 

 Brewster in his explanations and directions respecting illumina- 

 tion. I quote the following extracts : — " The principle of illumina- 

 tion on which Sir D. Brewster lays great, and we think fully 

 deserved stress, is that the focus of the illuminating rays shall be 

 coincident with the object so that there shall not be two sets of 

 rays at different angles, one proceeding from the luminous object 

 (source of hght ?) and the other from the object to be magnified." 

 Commenting further on the best means of developing this prin- 

 ciple, Dr. Carpenter says, " The rays of light proceeding from the 

 radiant point being brought to a focus by the ' condenser ' on the 

 object, cross each other there, and should proceed to the object- 

 glass of the microscope as if they came from the object itself. 

 Now unless they are made to converge upon the object at the same 

 angle at which they diverge to enter the objective, we cannot but 

 think that a source of error still remains, and that the most perfect 

 image possible formed by an achromatic object-glass of an object 

 which is artificially illuminated, can only be produced when the 

 rays from the som-ce of light take exactly the same course as if 

 they j)roceeded from the object itself. That they may have this 

 covu-se they must be made to converge upon the object by a con- 

 densing lens whose focus for parallel rays shall be the same as the 

 acting focus of the objective. A different condenser would thus 

 be required for every objective." 



But the practical experience of this able microscopist docs not 



* Quoted from Nageli and Schwendeuer, 2ud ed. p. 92. 

 t Todd's ' Cyclop. Anut.,' vol. iii. 1847. 



