Observations on the Tolles Ith. 63 



more than 82'^ of pencil with a balsam-mounted object, something 

 more has to be done, as obviously only 180^ could enter the front 

 surface of the slide. 



To give access of more pencil to the object I used the semi- 

 cylinder. This simple piece of apparatus applied to the front 

 surface of the slide dry, should, if the objective have 180^ of air 

 aperture, show 81° (-J-) of pencil on the cylindrical surface thereof. 

 Without the stop on the back such was the fact, viz. 81° at the 

 cyhndrical surface, and that being the internal angle of 180° of air 

 angle (or infinitely near that) at the plane dry surface of the slide, 

 as also at the plane front surface of the objective, when dry. Re- 

 placing the "stop" on the back surface, again no light passed to 

 the eye. A perfect eclipse existed. And next, to test the question 

 of more than equivalent pencil for 180'' external angle when the 

 object is immersed in balsam or other preservative medium, — air 

 was replaced with water above and below the slide containing the 

 object mounted in balsam. And now, darkness no longer, but a flood 

 of light illuminating the object with good definition of its features. 



The object actually used was a fine Ehomboides, of which the 

 cross-liHes were sharply defined, using an eye-piece about one inch 

 equivalent focus. Image-forming rays, evidently and certainly out- 

 side of the interior or balsam pencil of 180° external, and prac- 

 tically utilizing decidedly more than that. The balsam pencil 

 obtained was more than OS"". I might rest here and leave 

 Mr. Wenham to appropriate reflections over (so it appears to me) 

 his very hasty edict against my innovating ^-inch objective. 



But, as it seems rather necessary, I will state, as explanatory, 

 that I used ^{o" covering glass (rather less), and the systems were 

 but slightly closed from the extreme open-point. 



Very difierent indeed from the forced case he made of it. In 

 this matter a singular precipitating tendency to be either at one 

 extreme or the other extreme, fully "open" or fully closed, seems, 

 with my critic, a dire necessity when he assumes the judicial. 

 Mr. Wenham had not adjusted for maximum angle at all, nor 

 tried to. Just and merely " closed" to avoid " after quibbles." This 

 is verily amazing. How ohoni prior " quibbles," Mr. Wenham ? Who 

 is at fault? But to the point again. The. suggestion that O'OIS 

 focal distance as in the figure making necessary limit of angle to 118° 

 falls to the ground in view of the recited experiments. As already 

 stated, the objective (in the experiments described) was adjusted nearly 

 entirely open. This of itself annihilated the • 013" distance, sending 

 his figure to the realms of fancy. Not "abstruse," this teacher 

 encouragingly says. It is less than that, sir, and worse, — it is 

 simrious. There is no such thing in the case. With a dry object 

 mounted on the cover there is no distance involved (very possibly 

 indeed, and as will occur in everv such dry-mount). iVs there is 



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