CORRESPONDENCE, 135 



to be delivered not later tban Marcli 31, 1873, addressed — Secretary, 

 Natural History Prizes, 100, Fleet Street, E.G. 



N.B. — Importance will be attached by the Adjudicators to any 

 Notes and Records of Pond Life made during each month of the year 

 with reference to the Local Distribution, Development, or Hybernation 

 of the Species. 



The information might be sent in monthly, if preferred : such 

 information would be considered confidential. At the close of the 

 year the series of monthly reports thus sent in would be treated as 

 the competition for the prize. 



Adjudicators — Henry J. Slack, Esq., F.G.S., Secretary of the Eoyal 

 Microscopical Society, author of ' Marvels of Pond Life,' &c., &c. ; 

 Walter W. Eeeves, Esq., F.R.M.S., Assistant-Secretary to the Eoyal 

 Microscopical Society. 



COEEESPONDENCE. 



Peculiarities of Eesolution. 



To the Editor of the ^ MontWy Microscopical Journal.' 



Boston, Mass., U.S.A., Dec. 29, 1871. 



Sir, — I notice in your Journal for December that Mr. Wenham 

 admits that in the illustration I have given in your number for 

 November, that the rays are transmitted through the balsam-mounted 

 object " in all directions." Mr. Wenham doubts the possibility of pro- 

 ducing a set of combinations above the spherule to transmit a pencil 

 of even 90°. Of course the accomplistjueut of this last singularly- 

 supposed impossibility settles the question against his claim. 



Well, allow me to say I have had constructed a low-power dry 

 objective of clear focal distance of about y\j"^^-5 ^^^ ^^ 100° angular 

 aperture at the point for uncovered objects, — open point, in other 

 words. Before this objective I have placed " a little spherule " com- 

 posed of two hemispherical lenses, and a thin film of fluent blood 

 between. Easily and quickly done, no " cleverness." When the 

 blood is between in a fluent state, the angular aperture measures the 

 same as the dry objective alone. When the blood has dried, the angle 

 contracts to 82° {-{-). Now it is evident enough that the liquid blood 

 between the plane surfaces of the hemispheres opens the angle of 

 transmission through the sjiherule to about 120^, all rays beyond 

 being " totally " reflected. When the blood has dried, the ordinary 

 case of total reflexion at interior plane surfaces obtains, and the pencil 

 is cut down to 82° (-f-), i- e. incidence of 41° (-f-), at the 'plane surface 

 of the lower hemisphere of " the little spherule." 



Between Mr. Wenham and myself, of course this concludes the 

 case. But for the benefit of students fresh to practical optics, I 

 VOL. VII. L 



