208 ON THE THEOET OP THE MICROSCOPE. 



which all interpretation of microscopic vision has been hitherto 

 based as indisputable — nevertheless does not do so for a whole 

 class of objects and observations.' 



The occasion and purpose of this theoretical and experimental 

 enquiry, the leading points of which have been here noted, was^ 

 mainly a practical one — namely, to obtain some safe guide in deter- 

 mining a formulary of conditions concerned in the calculation of a 

 system of lenses ; but it has grown of itself to the dimensions of a 

 complete theory of the microscope, which touches every chapter of 

 micrographic doctrine, and has even added fresh chapters. In its 

 close connection with the technical construction of the microscope 

 this theory has proved serviceable in two ways. On the one 

 hand, the rigorous demands imposed by the practical aim of the 

 work have compelled investigations of a kind which no one would 

 have felt bound to undertake, merely because he was writing a 

 treatise on the microscope^ on the other hand, the actual con- 

 struction of microscopes on the principles deduced from theory 

 has brought into application the most sensitive tests to which any 

 theoretical considerations of this kind could be subjected. 



The details of these studies are communicated in full in the 8th 

 vol. of the Jena Journal of Medicine and Natural Sciences,"^' but a 

 condensed summary of results is now offered, in the hope that it 

 may prove acceptable to many practical microscopists . The same 

 order and method of enquiry is pursued here as in the more 

 detailed communication — namely the discussion first of all matters 

 that relate to the purely dioptric conditions ; and in the second 

 place the consideration of the new factors before mentioned, and 

 the share they take in the total optical performance of the micro- 

 scope. It must, however, be understood that the following 

 exposition is not a reproduction of the detailed studies elsewhere 

 given, and in no wise claims to be a full development or establish- 

 ment of the facts to be set forth. 



* The communication here referred to has been delayed by the illnesS' 

 of the author. Translator. 



