ADVANTAGES OF STEEEOSCOPIC BINOCULAR. 41 



plane being found to be much greater when the two pictures are 

 combined, than it is in either of them separately. In the absence 

 of any adequate Optical explanation of the greater range of focal 

 depth thus shown to be possessed by the Stereoscopic Binocular, the 

 Author is inclined to attribute it to an allowance for the relative dis- 

 tances of the parts which seems to be unconsciously made by the 

 Mind of the observer, when the solid image is shaped out in it by the 

 combination of the two pictures. This seems the more likely from 

 the second fact to be now mentioned : namely, that when the Bino- 

 cular 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 

 discomfort 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 examining, 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 (§ 121.) 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.* 



* It has happened to the Author to he frequently called on to explain 

 the advantages of the Binocular to Continental (especially German) 

 Sarans 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 con- 

 ception, they perceived no advantage in the Stereoscopic combination, 

 seeing such objects with it (visually) just as they had been previously 

 accustomed to see them (mentally) without it. But when he has ex- 

 hibited to them suitable objects with which they had not been previously 

 familiarized, and has caused them to look at these in the first instance 

 Monocularly, and then Stereoscopically, he has never failed to satisfy them 

 of the value of the latter method, except when some visual imperfec- 

 tion has prevented them from properly appreciating it. He may mention 

 that he has found the wing of the 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 peculiarly appro- 

 priate object for this demonstration ; the general inequality of its sur- 

 face, 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. 



