70 JOURNAL OF THE [April, 



objective which can be so used without breaking up the image 

 has yet to be made, in Europe at least. 



The slides from which I have taken my examples are two slides 

 of P. forinosuni and one of P. angulaiufii, mounted dry by 

 MoUer ; but I have verified my observations on a type-slide of 

 Pleurosigma mounted by Thum, also on dry mounts by the same 

 mounter and on a slide "of P. formosuvi mounted in phosphorus 

 by Mr. Stephenson, F.R.M.S., to prove there is nothing abnormal 

 about the structure. 



All my specimens have been tight on the cover, that the full 

 available aperture of the lens might be used on them ; and in 

 some instances, when there has been a doubt raised by others 

 whether some of the appearances might not be due to some of the 

 objects being off the cover, they have been tested by the vertical 

 illuminator. 



I have never taken a single photograph of any structure with- 

 out thoroughly working it out visually first, that every point 

 might be known to me before attaching the microscope to the 

 camera ; and each hour's work there means six hours at least of 

 previous investigation. 



The question is sometimes asked, " What is a true focus ? " and 

 I admit tfyit without certain landmarks the question is a puzzling 

 one, as the image will keep repeating itself both in and out of 

 focus. But to me those landmarks always exist, and in the struc-, 

 ture I have been working at will be found in the leading features, 

 such as the median line and the rows of larger perforations I have 

 before mentioned. Thus, in comparing slides Nos. 35 and 38, the 

 former gives what Mr. E. M Nelson calls the black dot, and the 

 latter the white dot ; but a little higher focus on each will reverse 

 them. The question then comes in, Which is right ? and the answer 

 is found in the median line with its two rows of perforations in 

 one case and the margins in the other. In both cases you can 

 reverse the image by raismg the focus, but at the same time you 

 lose the salient points of the valve. But, quite apart from this, 

 there is a crispness and brightness about the image when at the 

 true focus which cannot be mistaken by an expert. 



The image of diatom-structure with an apochromatic is abso- 

 lutely colorless, making the slides exhibited a true rendering of 

 the appearances in the microscope — that is, with all the upper 



