Royal Microscoincal Society. 177 



Dr. Woodward observed, speaking of photography ; but for direct 

 observations with the microscope, the effects obtained by them are 

 by no means to be compared with the marvellous results of a mono- 

 chromatic illumination. 



And I do not think it absolutely necessary for this purpose to 

 have recourse to a beam of the sun, which in many comitries less 

 favoured than Italy is not rarely a mere desideratum, and very 

 often a dim, cloudy thing. 



A brilliant luminous point of electric light — a light obtained 

 from oxhydrogenic flame — acting upon lime, magnesium, or zirco- 

 nium, perhaps also the magnesium- wire lamp, may supply the 

 deficiency of the sunbeam. Each of these, simply white lights, 

 decomposed through a prism, will give a monochromatic illumina- 

 tion sufiicient to reveal the best structural details, which up to 

 this day baffled the keenest researches of the student. 



[The following note has been sent to us by Professor Joseph 

 Gagliardi, who received it from Count Castracaue, and we print it 

 here because it has reference to the foregoing paper.] 



" As I am now settled in my new residence (near the Capitole), 

 I began three days ago to work in earnest. My newly self-imposed 

 task is to examine the Pinnularia genus, in order to ascertain if the 

 ribs (coste) or pinnules of the diiferent species be not truly con- 

 tinuous, but rather a series of dots close pressed together, in which 

 case the distinction between the two genera Navicula and Pinnularia 

 ought to be superseded. From this very moment I may say, without 

 fear of mistake, that P. ohlonga is distinguished with riblets (costole) 

 made up of a series of small lines (lineette) parallel one to the other, 

 and extremely minute — I was able to value their number to 41 GO to 

 a millimetre. Nevertheless, I cannot yet say I have recognized any 

 discontinuity in the pinnules of P. nobilis. As soon as I shall have 

 leisure to study with due attention, and with the monochromatic 

 illumination, the principal tyj)es of the said genus, I shall publish the 

 result in a special article. . . . 



" Yours affectionately, 



" Fbancis Casteacane. 



" EoME, ISth February." 



