216 On Iminersion Ohjeciives. 



Take, for example, the case of the single-front yVth belonging to 

 the museum, described above. When corrected for uncovered 

 objects its balsam angle is 65°, and it magnifies 460 diameters at 

 50 inches distance ; when closed to the point at which the Amphi- 

 pleura picture was made, it magnifies 620 diameters at 50 inches, 

 and its balsam angle is 87^. I hold that it would be as inaccurate 

 to measure the angle at some intermediate point and to call it the 

 maximum angle as to measure the magnifying power at an inter- 

 mediate point, and call it the maximum magnifying power. 



Lastly, with regard to the four-combination ith, described in 

 my June article. Mr. Wenham asks me to " allow that this addi- 

 tional lens embodies a deviation from the original question, which 

 was to the effect that there would be no loss of angle of aperture of 

 ordinary objectives by the immersion of the front surface in fluids." 

 I can only say that I so fully agree with Mr. Wenham's views on 

 this part of the question, that had I understood nothing more to 

 be in dispute at the time I wrote, I should never have written at 

 all; and if Mr. Wenham will now say that this is all he meant 

 seriously to maintain, and that he admits that a greater balsam 

 angle than 82' can be given to immersion objectives, as I have 

 endeavoured to demonstrate in this paper, it will give me great 

 pleasure to make the admission he desires ; but if he still adheres 

 to the contrary opinion, I cannot but hold that the 3^th in question 

 is a very excellent example of his error. 



This four-combination ith, by the way, seems to have given 

 great trouble to the Eev. S. Leslie Brakey. For him, indeed, the 

 unfortunate glass is not an objective at all, but an " optical machine" 

 (Query : Are not all modern objectives optical machmes ?), and 

 after giving his notions of what a real objective is, he states, quite 

 confidently, that " here the fourth ' system ' is not a system in this 

 sense at all. It is placed in front of an already perfect object- 

 glass," &c. 



I can only say to Mr. Brakey, that in this matter his thorough 

 misconception of the bearings of optical law on the question in dis- 

 pute has led him into the domain of the imagination (as well as into 

 the world of Greek poetry, with which he seems even more familiar 

 than with optics. This Journal, August, 1873, p. 99). 



In point of fact the front of the -^th in question is as much a 

 " system " as the single front of any other modern objective with 

 three systems. With the front at 12 inches distance from micro- 

 meter to screen the objective magnified without an eye-piece, 60 dia- 

 meters at the open point, 75 diameters when fully closed. Without 

 the front at the same distance it magnifies only 39 diameters at the 

 open point, and 42 when fully closed. With the front it works wet 

 and defines very well, without the front it works dry and does not 

 define very well. I have thought it worth while to take two 



