1892.] MICROSCOPICAL JOURNAL. 185 



lation, the actual refractive idex of any fluid. Another slip with 

 a concavity of different curvature would give results differing in 

 the strength of the correcting lenses and producing a different 

 mathematical formula without, however, altering essentially the 

 final results. It must be remembered also that crown glass itself 

 varies in refractive index in different specimens from 1.51 to 1.53, 

 and hence an optician mav send out an absolutely homogeneous 

 fluid when tested by his crown glass and yet it might show a re- 

 fractive index a little too low or a little too high when tested with 

 another specimen of crown glass. Prof. Smith says that an ab- 

 solutely homogeneous fluid cannot be obtained, but what may be 

 accepted as allowable variations from a theoretical homogeneous- 

 ness in such fluids when used with immersion lenses could be de- 

 cided only after further experimentation ; it seems to me that a 

 variation, using the device I have described, requiring a plus lens 

 of I. D. for neutralization or possibility of more than .^ D. 

 should condemn the fluid as unworthy of use with high-angled 

 objectives. 



51 West 47TH St., New York, yu>ie jo, iSqs. 



Note. —The D. in the foregoing is the abbreviation for Diop- 

 try, the unit of " strength " of spectacle lenses. A dioptry equals 

 I meter or approximately 40 inches. A spectacle lens of 2 diop- 

 trics is twice the strength of a lens of i dioptry — that is, it has a 

 focal length of 20 inches. To ascertain the focal length of any 

 lens designated in dioptrics divide 40 by the designating number 

 and the result is obtained in inches. Thus a lens of .2"^ dioptry 

 should be of 160 inches focus, but many opticians in the U. S. do 

 not adhere strictly to the true dioptric measurements in lenses of 

 very long focus, and the lenses marked .25 D. in the trial set used 

 in the above tests are really only of 144 inches focus. The .12 

 D. used is of double that focal length. 



When the plus sign is used it applies to convex lenses ; the 

 negative sign signifies concave lenses. 



Diatoms of the Connecticut Shore. — I. 



By W. a. terry, 



Bristol, Conn. 



In a paper published in this periodical December, 18S8, 1 gave a 

 brief account of the results of a search for diatoms on a small 

 section of the Connecticut shore near New Haven, and mentioned 

 particularly several forms new to me, and which the aid of experts 

 had not at that time enabled me to determine. In a review of 

 this paper in a subsequent number of the Tor7-ey Bulletin^ Prof. 

 Kain expresses the hope that I will follow this account with a 

 complete list of all the varieties found. I should have been glad 

 to have done this, but, unfortunately, the complete literature of 



