OPTICAL PRINCIPLES OF THE MICROSCOPE. 39 



110 w mentioned: namely, that when the Binocular is employed upon 

 objects suited to its powers, the prolonged use of it is attended with very 

 much less fatigue than is that of the Monocular Microscope. This, again, 

 may be in some degree attributed to the division of the work between the 

 two eyes; but the Author is satisfied that, unless there is a feeling of dis- 

 comfort in the eye itself, the sense of fatigue is rather mental than visual, 

 and that it proceeds from the constructive effort which the observer has 

 to make, who aims at realizing the solid form of the object he is examin- 

 ing, by an interpretation based on the flat picture of it presented by his 

 vision, aided only by the use of the focal adjustment, which enables him 

 to determine what are its near and what its remote parts, and to form an 

 estimate of their difference of distance. Now, a great part of this con- 

 structive effort is saved by the use of the Binocular; which at once brings 

 before the Mind's eye the solid image of the object, and thus gives to the 

 observer a conception of its form usually more complete and accurate 

 than he could derive from any amount of study of a Monocular picture. 1 



1 It has happened to the Author to be frequently called on to explain the ad- 

 vantages of the Binocular to Continental (especially German) savans, who had not 

 been previously acquainted with the instrument. And he has been struck with 

 finding that when he exhibited to them objects with which they had already 

 become familiar by careful study, and of whose solid forms they had attained an 

 accurate conception, they perceived no advantage in the Stereoscopic combina- 

 tion, seeing such objects with it (visually) just as they had been previously accus- 

 tomed to see them (mentally) without it. But when he has exhibited to them 

 suitable objects with which they had not been previously familiarized, and has 

 caused them to* look at these in the first instance monoeularly, and then stereo- 

 seopically, he has never failed to satisfy them of the value of the latter method, 

 except when some visual imperfection has prevented them from properly appre- 

 ciating it. He may mention that he has found the wing ef the little Moth known 

 as Zenzera (Esculi, which has an undulating surface, whereon the scales are set 

 at various angles, instead of having the usual imbricated arrangement, a pecu- 

 liarly appropriate object for this demonstration. The general inequality of its 

 surface, and the individual obliquities of its scales, being at once shown by the 

 Binocular, with a force and completeness which could not be attained by the most 

 prolonged and careful Monocular study. 



