38 THE MICROSCOPE AND ITS REVELATIONS. 



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 dis- 

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

 nearer to or more remote from the objective. Now, as will be explained 

 hereafter ( 158, n.), increase of the angle of aperture is necessarily 

 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 to it 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. 



40. In concluding these general observations upon the use of the Ste- 

 reoscopic Binocular, the Author would draw attention to two important 

 advantages he has found it to possess; his own experience on these points 

 being fully confirmed by that of others. In the first place, the penetrat- 

 ing power or focal depth of the Binocular is greatly superior to that of the 

 Monocular microscope; so that an object whose surface presents consider- 

 able 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 00 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 separately (the prism being in place), and then with both eyes 

 together; the distinctness of the parts which lie above and beneath the 

 focal 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 distances of the 

 parts, which seems to be unconsciously made by the mind of the ob- 

 server, 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 



half-inch Objective of 40 aperture resembling the one first constructed to his 

 order by Messrs. Powell and Lealand, and now procurable from several excellent 

 makers in the study of Polycystina, the smaller Foraminifera, or the larger 

 discoidal Diatoms, viewed as opaque objects^soon becomes sensible of its advan- 

 tage over Objectives of the same power but of larger angular aperture, in giving 

 (1) unexaggerated. relief, (2) much greater focal depth, and (3) such a working 

 distance as enables side-illumination to be conveniently used. Having lately had 

 occasion to give much attention to the structure and development of Isthmia 

 (Chap, vii.), the writer has found great advantage from the use of a 1-4 th 

 objective, constructed by Zeiss, of what will be considered by many the absurdly 

 low angle of 40; the truth of the conception it gives of the solid forms of the 

 frustules (when viewed as opaque objects), which is capable of easy verification, 

 being in striking contrast with the violent exaggeration of relief which is pro- 

 duced when the same objects are similarly viewed through a l-4th inch of 90 or 

 130 aperture. Doubtless the elementary structure of the frustule can only be 

 properly studied by an Objective of large angle; but this is an altogether different 

 inquiry. 



