Universal Aper to meter. 203 



Figs. 9 and 10. A long-focus lens may be applied at the eye- 

 hole if necessary, or the supplemental tube recommeded for 

 Abbe's apertometer, but I find all that is required is my ordinary 

 reading glasses, and I can make the bisection of the little 

 bright circle as accurately as when using a compound micro- 

 scope to view it, even with a i-i2th objective. 



As already remarked, I think that in sharpness of the image 

 the spherical surface will be found superior to the cylindrical. 

 The results obtained with the instrument used in this way 

 agree with those given when it is used as I have already de- 

 scribed. The same exaggerated reading of low angled long, 

 focus objectives is to be noticed as when using the bull's eye 

 without the lines. The Spencer 2-3d inch gave, using the in- 

 strument as Abbe's apertometer, 36°.* With wide angles and 

 high powers, the anomaly disappears, and the agreement be- 

 tween the two methods is as close as could be desired; the 

 use of the lines with the bull's eye gives, however, a superiority 

 to my own method of measuring, at least, for objectives of 

 long focus, but there is certainly very great ease in reading 

 the angle, using it after the manner of the apertometer of 

 Abbe, though not more than in my own; the spot of light 

 with wide angles and short foci disappears promptly or is 

 readily brought to the intersection of the lines. 



If we remove the bull's eye slide and focus on the lines on 

 the plain slide, we can obtain the air angle with great accuracy 

 by simply looking through the eye-hole in the cap applied at 

 the end of the tube (/', Fig. 2), and observing, by aid of a 

 long focus lens, when the image of the hole in / is neatly 

 bisected on either side, as shown in figures 10 and 11. If, 

 instead of a single lens, a short compound microscope with an 

 objective of 4 inches focus is used, the eye-hole will not be 

 necessary. I do not, except for higher amplification, see any 

 need of this supplemental part. The highest air angle objec- 

 tive I have yet measured, is an old jV^h by Spencer, 163°. I do 

 not doubt that some of the first-class modern dry objectives of 

 recent date may reach perhaps 170° or more; but even with an 

 angle of 163° the extreme rays strike with such obliquity that 



*This is the glass angle, and 12° too much ! As I have not by me one of Abbe's apertometers 

 I cannot say whether that would give this exaggerated angle, 36° in glass, which my instrument 

 gives when used after the manner of his apertometer, but I cannot see why it should not ; the 

 principle is the same ; the use of the intersecting lines will, however, in my method, give, as 

 already shown, 24" for the true angle of this objective in glass, or 37" as air angle. 



