ON THE THE3ET OF THE MICROSCOPE, 225 



as are readily obtained with objectives of -J3- incli, and seem 

 really serviceable — are nevertheless not available in practice, 

 because various other conditions besides the perfection of the 

 image-forming function cannot be fulfilled. The final inference 

 from these data is that improvement of the microscope should no 

 longer be sought for by aiming at still higher magnifying power 

 and amplification, but rather at a more correct performance of the 

 middle and moderately high jDowers. It will be a real advance 

 of the optician's art, and of infinite service to the scientific 

 use of the microscope, when we succeed in accomplishing 

 with objectives of -J-th and ^th what is now only attained 

 with mnch higher powers. Such an aim is within the range of 

 what is possible ; other aims will only be directed at castles in 

 the air ! 



X. In the account of Professor Abbe's researches, to be communi- 

 cated in the JenaJournal^^ to which reference has beenbefore made, 

 new and exact methods are given by which every determinable 

 point in the construction of the microscope e.g , focal length of 

 each lens, angle of aperture, character and limits of objective and 

 ocular functions may be empirically ascertained. ; and, in 

 addition to this, a mode of procedure is described which renders 

 it possible, with very simple means, to examine in instruments 

 already made, every fault of definition of image indicated by 

 theory, and thus to determine their relative excellence. The 

 methods commonly recommended for testing the state of spherical 

 and chromatic correction of the objective are not adequate to the 

 actual requirements of the case, and quite fail to explain the true 

 character of the aberrations ; for the eff'ect of these, as rendered 

 visible by such methods, is not due to elementary sources, but to 

 combined results of many difi'erent causes ; and since each separate 

 source of error plays a very unequal part in the total result, any 

 judgment founded on this alone, may, according to circumstances, 



* While this is passing through the 'press, I learn that the proposed 

 communication is deferred, and that a separate treatise on the subject will 

 be published by the author.— H.E.F. 



