214 On Immersion Objectives. 



It could also be shown in the same manner that by moving the 

 points F' and F" still nearer to the front, still greater pencils than 

 100^ from balsam, and 122° from water, could be transmitted. I 

 have selected the balsam angle of 100° for the demonstration 

 because I shall presently describe an immersion rijth made by Mr. 

 ToUes, which has this balsam angle. 



It seems almost unnecessary to add that the divergence of the 

 extreme rays from the optical axis after their emergence into air 

 behind the front might be diminished somewhat, and the aberra- 

 tions of the front partly corrected, by substituting a properly con- 

 structed triple front for the single one above discussed. This is a 

 mere matter of detail for the consideration of the maker, and does 

 not affect the principle. 



Next with regard to the possibility of correcting the aberrations 

 of the divergent pencil transmitted backward from the front of the 

 objective, and of forming a distinct image for the eye-piece when 

 the balsam angle exceeds 82°. This question, I respectfully submit 

 to my friend, Mr. Wenham, is not one which can be decided a priori 

 by considerations of optical law. The devices of ingenious workmen 

 are very numerous, and are generally kept secret. Hence mathe- 

 matical analysis has not yet been applied to the discussion of the 

 question in such a way as to enable us to predict all the possibilities 

 of the case. Under these circumstances, all that we can do is to 

 examine with candour any particular device submitted for our con- 

 sideration, and to measure experimentally with suitable precautions 

 the results attained. 



In this spirit I have examined, since I wrote my first paper, 

 two objectives constructed by Mr. ToUes, in which the balsam angle 

 exceeds 82°, and in which the corrections are effected by two 

 posterior combinations only. 



The first is an immersion roth, specially constructed for the 

 Army Medical Museum. This is an objective of three systems, the 

 front being a single nearly hemispherical lens of crown glass, very 

 similar in form to the one discussed above, and not very difierent 

 in dimensions. It works as an immersion glass only. Its balsam 

 angle measured by the method described in my last paper is 65° at 

 the open point ; at the point of maximum, which is reached before 

 the screw collar is fully closed, it is 87°. I call attention to the 

 fact that by my method the angle measured is that transmitted into 

 solid balsam ; into balsam " of the same index as crown glass," it 

 would be a trifle greater. Now this objective performs charmingly 

 at every position of the screw collar from the open point to the point 

 of maximum angle, provided the thickness of the cover is exactly 

 suited to the adjustment. Indeed I am compelled to give it the 

 preference, for exquisite definition, over any objective of the same 

 power that ever passed through my hands. At the point of maxi- 



