504 Transactions of the Society. 



standpoint is the direct co-ordination of the illuminator and 

 objective considered as ixirts of the same instrument tending hy one 

 combined series of acts to a common result in which the function 

 of each part is a complement of the other. 



But it may be objected that a theory of illumination is better 

 rej^laced by the p7'actice which has long been successfully carried 

 out by force of instinct aided by a little empirical manipulation, and 

 that every microscopist can thus gain sufficient insight and mastery 

 of his instrument to become independent alike of the restraints of 

 theory and the methods of science. Or that, if inclined to rest his 

 practice on a firmer foundation, he has only to turn to some 

 " manual " and find therein all needful guidance. Such, indeed, is 

 the general belief; and he who dissents from it in the face of 

 current opinion and " authority " will scarcely escape the imputation 

 of prejudice or presumption. Nevertheless I put in my plea of 

 " not guilty," and proceed to offer reasons for dissent from the 

 popular creed. 



1. The microscopic image cannot be interpreted by mere 

 intuition or insight of indisputable self-evident realities, but is the 

 outcome of a prolonged study of optical phenomena occurring under 

 unusual conditions, and presented to the eye in an unusual form. 

 Visual perception may be correct as regards the optical efi'ects 

 presented, but this does not ensure accuracy of inference regard- 

 ing the objective facts which are their antecedent. Experience 

 teaches us that a perfect optical image requires a critically perfect 

 illumination of the object ; but a knowledge of optical effects as 

 complete as exact science can make it is required before this 

 illumination can be effected with certainty and constancy. No 

 amount of practice will confer the power of discriminating between 

 adventitious effects (which are yet true in an optical sense) and 

 those which really characterize some structural detail of the object. 

 The fact that a clearly seen and perfectly defined image is in a 

 large number of cases not conformable with the object itself has 

 been demonstrated beyond possibility of dispute.* 



2. Yet our handbooks have nothing to say on these matters 

 which can assist even the novice in his theoretical studies. The 

 ordinary schoolboy crib from some elementary treatise on dioptrics 

 transferred with no little parade to the introductory chapters of 

 our handbooks as optical principles of the Microscope (!) is simply 

 an evasion of the obhgation (confessed, indeed, in the attempt) to 

 explain the formation and character of the Microscope image. 

 And. so far as the theory of illumination is concerned, catoptric 

 problems of fundamental significance, such as the action of plane 



* For further observations on this subject I beg to refer to various articles 

 in the Bristol Naturalists' Society's ' Proceedings,' and particularly to an essay on 

 the question, '■ Is there a Science of Microscopy ? " 



