On the Navicula Spencerii. 269 



upon any mind, that rows of prominences, variously arranged, 

 produce all the appearances of curved or rectilinear stria* seen on 

 these objects, and nothing which I have yet seen on the Navicula 

 Spencerii is at variance with the view. 



If the lines seen on this species or upon the N. Hippocampus 

 are due to rows of pits or perforations, I cannot see why they 

 should not appear equally distinct and strongly marked whether 

 viewed in the longitudinal or transverse directions, a fact which 

 is easily explained by referring the appearances to two sets of 

 grooves differing in depth, forming rows of prominences by their 

 crossing each other. Mr. Spencer agrees with me in believing 

 that all the lines on the Naviculacese are due to elevations or 

 prominences, and these he states differ very much in form as well 

 as distribution, in different species. I believe these prominences 

 will be found on every species of the Naviculaceae no matter 

 how small, and that any species on which they cannot be made 

 out will be a reproach to the microscope of the age. 



I now proceed to give a few extracts from letters received by 

 nie from Mr. Spencer, referring to his first examinations of the 

 ■IV Spencerii and his subsequent efforts. They appear to me to 

 be of much interest, as illustrating his determination to reach the 

 very first order of excellence, and an unwillingness to rest satis- 

 fied with what might justly have been considered splendid suc- 

 cess. In his letter to me, dated Dec. 28, 1847, he informs me 

 °f his first reception of the N. Spencerii which I had sent to 

 him. telling him that he mast resolve at least two sets of lines 

 upon it, although I could do nothing with it with my Chevalier 

 lenses. He says, "I find the Navicula is striated, as you sup- 

 pose, longitudinally and transversely. It required all the means 

 au< i appliances I could command to resolve it. But I hope the 

 powers I am now making will tell a different story." In several 

 subsequent letters he remarks on this test as being too easy, al- 

 though the most difficult one known ; and in a letter, dated Au- 

 gust 23, 18 18, he says, " I hope we shall soon be able to find a 

 much more difficult object." In a letter, dated Nov. 4, 1848, he 

 remarks on the ease with which the lines could be resolved on 

 the objects mounted dry, and adds, "you have done more than 

 I expected your objective would enable you to do, in resolving 

 both systems of lines at once. My late lenses will do this readily 

 °n all the specimens I have tried. With a T V nearly completed, 

 I observe the cross lines by central light by my hand tube, I 

 fnist to be able ere long to make out both systems by the same 

 illumination." Finally, under date of Nov. 20, 1848," he an- 

 bounces the discovery of the long desired test object, which 

 should exceed the N. Spencerii in difficulty. He says, "you will 

 ^member my calling your attention to a small shell among the 

 infusoria from guano, and that an examination of the species re- 



