40 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



practical grounds — that when an Objective of -wider angle than 40° 

 is used with the Stereoscopic Binocular, the object viewed by it is 

 represented in exaggerated relief, so that its apparent form must 

 be more or less distorted. — There are other substantial reasons, 

 moreover, why Objectives of limited Angle of aperture should be 

 preferred (save in particular cases) for use with the Stereoscopic 

 Binocular. As the special value of this instrument is to convey 

 to the mind a notion of the solid forms of objects, and of the 

 relations of their parts to each other, not merely on the same but 

 on different planes, it is obvious that those Objectives are most 

 suitable to produce this effect, which possess the greatest amount 

 of penetration or focal depth, that is, which most distinctly show, 

 not merely what is precisely in the focal plane, but what lies nearer 

 to or more l-ernote from the Objective. Now, as will be explained 

 hereafter (§ 131, n.), increase of the Angle of Aperture is neces- 

 sarily attended with diminution of Penetrating power ; so that 

 an Objective of 60° or 80° of aperture, though exhibiting minute 

 surface- details which an Objective of 40° cannot show, is much 

 inferior in suitability to convey a true conception of the general 

 form of any object, the parts of which project considerably above 

 the focal plane or recede below it.* 



31. In concluding these general observations upon the use of the 

 Stereoscopic Binocular, the Author would draw attention to two 

 important advantages he has found it to possess ; his own expe- 

 rience on these points being fully confirmed by that of others. In 

 the first place, the Penetrating poiver or Focal Depth of the Bino- 

 cular is greatly superior to that of the Monocular Microscope ; so 

 that an object whose surface presents considerable inequalities, is 

 very much more distinctly seen with the former than with the 

 latter. The difference may in part be attributed to the practical 

 reduction in the Angle of Aperture of the Objective, which is 

 produced by the division of the cone of rays transmitted through 

 it into two halves ; so that the picture received through each half 

 of an Objective of 60° is formed by rays diverging at an angle 

 of only 30°. But that this optical explanation does not go far to 

 account for the fact, is easily proved by the simple experiment of 

 looking at the object in the first instance through each eye sepa- 

 rately (the prism being in place), and then with both eyes together ; 

 the distinctness of the parts which lie above and beneath the focal 



* In accordance with these principles, the Author has caused Messrs. 

 Powell and Lealand to construct for him an Objective of Half -inch focus 

 with an Angular aperture of 40° ; and he has found it to answer most 

 admirably the purpose for which it was intended, — the examination of 

 Opaque objects with the Stereoscopic Binocular. For not only are these 

 represented in their true forms, but the relations of their different parts 

 are seen with a completeness not otherwise attainable. And an Objective 

 so constructed has this great advantage over one whose originally large 

 aperture has been reduced by a diaphragm, — that the distance between 

 its front lens and the object is so much greater, as to admit far more 

 conveniently of side Illumination. 



