12 Measurement of Immersed Apertures. 



simply because there was no refraction. The balsam object was 

 not magnified. It occupied a like focal distance to the dry ones 

 outside, and the same adjustment served for either. 



Here I had directly solved the problem of securing the full dry, 

 or the same aperture on a balsam-mounted object. This was done 

 eighteen years ago, and the experiments are described in the ' Quart. 

 Journ. of Mic. Science,' No. XII., July, 1855. Other lenses were 

 used upon balsam-mounted objects, under covers, but the radii 

 being less by the thickness of covering glass, the object still occu- 

 pied the diametrical plane.* As it may be doubted whether I 

 should, for a mere demonstration, undertake the excessively difiicult 

 task of making a number of almost invisible lenses, I can reply, 

 that though an amateur, I disliked needless trouble, and therefore 

 made this an easy matter ; and as from the practical nature of 

 microscope work, scraps of such information are considered refresh- 

 ing by some of our members, I append a separate description. It 

 is clear that this adaptation is similar in principle to the four-com- 

 bination lens sent to Dr. Woodward. I hope that I may be ac- 

 quitted of attempting to claim everything, and therefore leave to 

 Mr. Tolles the honour of proving whether such a lens will be of 

 practical use to microscopists, in viewing such tests as are mounted 

 in balsam. I trust that Col. Woodward, having affirmed that 

 " the position taken by me is certainly true for objectives as ordi- 

 narily constructed," will allow that this additional lens embodies a 

 deviation from the original question, which was to the effect that 

 there would be no loss of angle aperture of ordinary objectives by 

 the immersion of the front surface in fluids, and I conclude by 

 thanking him for his impartial aid, in bringing facts so near at last, 

 thus ending this interminable question, which I fear must have 

 become very wearisome to the readers of the ' M. M. J.' 



How to Make the Atomic Lenses. 

 The plano-convex and hemispherical lenses referred to were made 

 as follows : — Strips of thin clear window-glass were drawn out into 

 threads with the blow-pipe flame, a portion was then held in the 

 point of the flame and fused into a spherule of the desired size. A 

 number of these may be formed in a short time. The spherical 

 figure is pretty accurate up to one-twentieth of an inch in diameter. 

 One precaution must be observed. The strips of glass from 

 which the threads are drawn must be broken and not cut of with a 

 diamond, if so, the spherules will not retain a clear polished surface, 

 as the rippled cut of the diamond leaves its mark to the last. The 

 blow-pipe may be an ordinary portable one, and the flame of a 



* If the fourth lens is used witli a vmter continuity, the object will not occupy 

 the central or diametrical position, but a small distance within it, in accordance 

 with the law of displacement. 



