30 JOURNAL OF THE [February, 



On B is cemented a stop, formed of a piece of paper black- 

 ened with lamp-black in shellac. The diameter of the central 

 stop is TTnrths of an inch. The width of the annular opening 

 around the stop is Ath of an inch. 



Each of the lenses in the experimental form of the illumina- 

 tor exhibited has a diameter of i^ inches. It is evident that 

 this diameter may be lessened in the lenses B and C, so that the 

 combination when mounted will have the form of the frustum of 

 a cone. With this form, the combination could enter the aper- 

 ture in the majority of microscope stages, and its upper lens be 

 brought even in contact with the under side of a slide. 



The mean angle of the emergent rays at the upper lens, C, is 



The mean diameter of the annular opening of the stop is cal- 

 culated in reference to the curvatures of the lenses, so that the 

 central rays issuing from this stop fall normally on the convex 

 surface of the lens C, and thus traverse it without refraction. 

 This also tends to correct the chromatic dispersion of the pen- 

 cil of rays emerging from B, whose boundaries of red and blue 

 fall in directions inclined towards the normal of the lens C, on 

 opposite sides of this normal. 



This combination is not patented, and is at the service of all 

 opticians and microscopists. 



VITALITY OF THE LARVAE OF THE NUT-WEEVIL. 



BY F. W. LEGGETT. 



{Read Jan. \c,th, 1886.) 



In preparing spiracles for mounting, I have made considerable 

 use of those furnished by the nut-weevil {Balafiinus nucum), a. 

 plentiful supply of which I found, much to the disgust of my 

 family, in some hickory nuts on our table. It is not appetizing, 

 I must acknowledge, to find one of these grubs ensconced within 

 a morsel of fruit which you are about to convey to your mouth, 

 and very few people are educated up to the point of gazing with 

 enjoyment on its white, squirming body ; yet its. whole life- 

 history is interesting — a fact too well known to need repetition. 

 I knew that, shut up within its air-tight hickory-house, its con- 

 sumption of air must be infinitesimal, but I was not prepared to 



