Intro.] INTRODUCTION 5 



for it produces an image wholly independent of the eye of the ob- 

 server. This image may be fixed on a photographic plate or used as 

 a basis for a drawing (fig. i). On the other hand, when used as a 

 microscope in the ordinary way, the eye of the observer is an integral 

 part of the optical combination, just as integral a part as the objec- 

 tive or the ocular (fig. 2, 3). This being the case the optical perfec- 

 tion of the eye is as influencing on the final retinal image as the 

 perfection of the other optical parts. 



And finally, quoting again from the preface of the third edition, 

 "In considering the real greatness of the microscope and the truly 

 splendid service it has rendered, the fact has not been lost sight of 

 that the microscope is, after all, only an aid to the eye of the observer, 

 only a means of getting a larger image on the retina than would be 

 possible without it, but the appreciation of this retinal image, whether 

 it is made with or without the aid of a microscope, must always 

 depend upon the character and training of the seeing and appreciating 

 brain behind the eye. The microscope simply aids the eye in furnish- 

 ing raw material, so to speak, for the brain to work upon." 



