64 Observaiions on the Tones' \th. 



no distance, you are invited to "plain measurement" of whatever 

 triangle in the case. Deny if you can. 



Fui'thermore, anyone having under such covering glass a dry 

 object in view with this (London) |-inch, when it is ad.justed to 

 good definition of the object by extremest obliquity of the radiant- 

 light, and using this light directly, i.e. without intervening con- 

 denser of any sort, — then, it will be found, that the extremest 

 obliquity that can he made incident upon the bottom surface of the 

 slide will enter the objective, and it will be manifest enough that 

 the only hmit of obliquity of incidence of illuminating pencil is 

 parallelism with the surface of the slide. 



Prove (do not fail to do that) that the most oblique rays actually 

 traverse the object and constitute an image of it at the eye-piece. 

 How ? — by simply passing a card, or like thin-edged body, upward 

 toward the slide until the very thinnest wedge of hght passes between 

 the edge of it and the slide, or the stage, as the case may be. (Plainly 

 for this purpose a slide-holder helow the stage is necessary to reach 

 the extremest angle.) 



On my way South last March, the Journal for that month 

 came into my hands, and I read Mr. Wenham's article with no 

 little surprise, and I determined that on my return I would imme- 

 diately subject the -|-inch to the " stop " proof, which I knew right 

 well would be conclusive. Meantime I could only send the illus- 

 tration and seemingly necessary instruction how to make maximum 

 angle at open point, while diminishing it with closure of the 

 systems. Not netv in my practice by any means. 



And in this connection I will gladly assure Mr. Wenham that 

 he is not being " hoist of his own petard." Not so bad as that. 

 The bitter reflection may be dismissed. He has had nothing to do 

 with the machinery of the hoisting, if that operation is going on ; 

 has not furnished ammunition, nor plan, in any particular. I have not 

 even read his " writings " referred to. 



I invite Mr. Wenham's plainest comments, without considera- 

 tions of any sort other than to settle the point at issue, viz, practical 

 objective angle over 82^ for objects mounted in balsam or other 

 preservative media. 



Business, business relations, " income," absence, or absent 

 health, need not stand in the way or affect the discussion. 



Since writing the above, I have noted more particularly Mr. 

 Wenham's remarks about " getting no further than the front lens " 

 with a diagram. 



Now, in the paper sent you last month, the back and middle 

 systems not given are assumed to be of angular aperture = 60°. I 

 did not mention, what is true, viz. the " Museum " xV-inch to 

 which Dr. Woodward ascribes 87'^ hard-balsam angle, has, inner 

 systems, back and middle, of 62^° angle. Now this is good as a 



