162 Transactions of the 



practical difficulties in the case of those objects which can be dis- 

 tinctly observed. Dr. Pigott seems to have fallen into this error 

 with regard to other objects than Amjihijyleura pellucida. Thus 

 for example, he says :* — " It will be extremely interesting to hear 

 that our American brothers can resolve the striae of known diatoms, 

 having them arranged from 120,000 to 150,000 lines to the inch." 

 This is a rather sweeping statement, and in the absence of a list of 

 the " known diatoms" whose striae are estimated at this figure, 

 I wiU confine my remarks to the Amjphipleura pelhccida. Prichard 

 states the length of this diatom at from -rrirth to sho^h. of an inch. 

 The ' Micrographic Dictionary ' gives the mean length at "0044. The 

 fnistules on the two sHdes sent me by Powell and Lealand measured 

 from TTirth to ^roth of an inch. 



From a number of negatives, I send prints of two. 



No. XIX. Photograph of Amphipleura pellucida; 960 dia- 

 meters. Frustule ^o-xrth of an inch long. 



No. XX. Another view of the same frustule ; same power. 



After making the photographs, I carefully counted the striae, 

 both on the glass negatives and in the microscope. The frustule 

 selected for the above photographs counted 91 striae to the 

 thousandth of an inch. Larger frustules exhibited rather coarser, 

 smaller ones rather finer striae. Oji the smallest frustules at my 

 disposal, several of them only wcr^h of an inch in length, I found 

 no example in which the number of striae exceeded 100 to the 

 thousandth of an inch. The striae of these smallest frustules do 

 not then rival in fineness the nineteenth band of the Nobert's plate, 

 the study of which I am convinced will make those microscopists 

 who are able to resolve it more careful in their "estimates," vdth 

 regard to the striae of diatoms. 



In conclusion, I have to add, simply, that all the photographs 

 forwarded with this paper were taken by the immersion -^ih. of 

 Powell and Lealand, with the exception of the Cbscinodiscus, for 

 which I employed the immersion -^th of the same makers. 



Note. — Since writing the foregoing my attention has been 

 directed to the letter of Mr. Lobb,t from which it would appear 

 that i\[r. Lealand, of the firm of Powell and Lealand, "has suc- 

 ceeded in counting the Amphipleura lines, and finds them 100 in 

 •rw?rth of an inch." -This agrees perfectly with my own results 

 on the smallest frustules. I am happy to find that my testimony 

 against the extravagant " estimates '' which have been published, is 

 corroborated by such a witness. 



[Plates LXXIX. and LXXX. contain selections from the photo- 

 graphs sent to the Society by Dr. "Woodward. — Ed. M. M. J.J 



* 'Monthly Microscopical Journal,' vol. iii., p. 311. t lb., vol. iii., p. 104. 



