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XXVIII. — On the Theory of Illuminating Apparatus employed 



ivi'th the Microscope. Part I. 



By Dr. H. E. Fmpp, Ex.-Off. F.K.M.S.* 



(Bead llth June, 1879.) 



Whoever tries to explain the diverse practice of microscopists in 

 illuminating difficult objects, will find it equally hard to reconcile 

 the conflicting opinions upon which this practice is based, and the 

 contradictory results obtained by the methods employed. And 

 the question will naturally arise in his mind, whether all micro- 

 scopists accept in the same sense, and with the same confidence, 

 those optical principles whose validity is beyond question, but whose 

 practical import depends entirely upon their correct application. 



For when the performance of the different kinds of illuminators 

 is under discussion, the sincerity of appeal to optical principle often 

 seems so dubious that one cannot but be impressed with the con- 

 viction of the expediency of reconsidering the whole theory of the 

 action of the various apparatus now in use. 



If there be no common ground of accepted doctrine it is 

 obviously impossible to institute precise comparisons of the several 

 excellencies, defects, or relative fitness for the particular purpose 

 for which each illuminator is used (or designed), until some 

 general rationale of their optical action has been established, and 

 the more or less perfect realization of this optical rationale has been 

 demonstrated for each case. My aim, therefore, in the present 

 paper is to systematize as far as lies in my power those general 

 propositions which, if accepted as rightly representing the optical 

 aspects of the subject, may be used as a basis and standard of such 

 comparisons. But while it is necessary to keep in view the various 

 modes of action of the various apparatus used, in order to draw the 

 particular conclusions upon which general propositions may be 

 founded, I purpose to avoid all side issues involved in the attempt 

 to decide for or against the claims severally advanced by the 

 advocates of one or the other illuminator. It would indeed be 

 impossible to include within prescribed limits a reasoned estimate 

 of the performance of each separate invention, and equally fruitless 

 to enter upon bypaths of theory which only interest those who are 

 studying special objects and employing unusual methods of pro- 

 cedure according to the requirements of the illuminator used or 

 object examined. Any inferences here drawn on general theoretical 

 grounds respecting the real worth and use of the several classes of 

 illuminators, will turn not upon art and skill of construction, but 

 upon the optical intention and actual fulfilment of service in 

 perfecting the definition of the Microscope image. In a word, my 

 * President of the Bristol Naturalists* Society. 



