268 On the Navicula Spencerii. 



balsam, without any particular management of light beyond what 

 is attainable by the ordinary reflection. By the aid of Ross's 

 T ^th we have unmistakeably brought out these markings on an 

 object mounted dry, and by direct light through the achromatic 

 condensor, and moreover resolved these lines into dots or granula- 

 tions. I am prepared to say that should you be able to effect 

 this, it will at once stamp Spencer as one of the first makers 

 either in the old or new world. I ought however to observe here, 

 that I think that ere long I shall be enabled to report the resolu- 

 tion of these markings by direct light even in the object mounted 

 in balsam, having pretty nearly accomplished it in the last trial 

 with a friend." 



As Spencer long ago demonstrated the existence of both the 

 longitudinal and transverse lines, it followed of course that if both 

 sets could be seen at once, the surface would appear broken into 

 little squares or granulations, precisely as in the much larger and 

 easier test the N. Hippocampus. The resolution of these tests 

 mounted dry is so much easier than when in balsam, that objects 

 thus mounted are of little value in testing the powers of lenses, 

 although they may answer well when the end is to make out 

 the real structure of the object itself. 



I proceed now to notice a discrepancy between Mr. De la Rue's 

 measurements and my own, and to remark upon the existence of 

 the depressions or perforations which he claims to have seen. I 

 have carefully and repeatedly measured by means of Chevalier's 

 camera lucida eye-piece, the distance between the transverse lines 

 or rows of prominences on the adult Navicula Hippocampus, and 

 have invariably found it to be about ?Tr ^ r ,th of an English inch, 

 and both by estimation and the most careful measurement I could 

 make on an object so minute, the corresponding lines on the 

 Navicula Speucerii are, as I stated in this Journal, at least three 

 to five times closer, or from rt Vtir» to *^Viv ° f an inch apart 

 instead of „ i- s to T7 n as stated by Mr. De la Rue. To de- 

 cide whether the appearance of lines on the N. Spencerii is due 

 to the allinement of depressions or perforations, or to rows of ele- 

 vations, or finally to two or more sets of grooves which by their 

 crossing break its surface into granulations, is not an easy task if 

 we confine our attention to so small an object as the N. Spencerii, 



pe 



in all the Naviculaceae (connected perhaps with their mysterious 

 organs of locomotion), and the real nature of this structure can 

 best be made out by studying the forms which are most coarsely 

 marked, such as some of ihe species of Rhaphoneis. Sceptroneis, 

 Cocconeis, Stauroneis and the large species of the genus Navicula, 

 such as the N. Baltica and N. Hippocampus. If attention is 

 paid to the direction of the shadows, and the deceptions due to 

 refracted light are avoided, I believe that little doubt will be left 







