ON THE THEORY OP THE MICROSCOPE. 261 



tion to review the whole subject of illumination of objects for the 

 microscope in another paper and at a later period. I here only- 

 direct attention to a translation, published in the Monthly 

 Microscopical Journal (April, a.c."), of an article written by Pro- 

 fessor Abbe which appeared in Schultz's Archive (at the same 

 time with the longer essay here translated), and which gives an 

 account of an illuminating apparatus invented by the author. In 

 the English journal, however, an introductory discussion of 

 optical principles, (written by the author and referring to what 

 had been written in his former communication), is omitted. There 

 is still ample room for a general review and discussion of 

 the whole question. The practical importance of suitable 

 illumination for all classes of objects has been fally shewn in this 

 essay, and apart from all theoretical considerations, it is no 

 slight evil that the time notoriously wasted in trying useless 

 combinations and methods of illumination should result, as not 

 unfrequently happens, in the discredit of the best qualities of an 

 objective, or what is indeed a still more common case, an objective 

 is credited with qualities which it does not possess, but which if 

 it did would be of very questionable value. Until, moreover, the 

 the optical principles by which the construction of illuminating 

 apparatus should be regulated are definitely formularised and 

 generally acted upon, condensers and illnminating apparatus — 

 some more or less serviceable, others useless or mischievous — 

 will continue to be made and pressed upon public attention, to 

 the further confusion of a matter that has already suffered too 

 many false issues. 



