CORRIGENDA ET ADDENDA. 



P. 43, note.] Mr. Rylands calls attention to the fact 

 that in treating of the "penetrating power " of the micro- 

 scope, much error prevails, and authors on the subject 

 too frequently confound it with definition or angular 

 aperture. The reader is referred to " Microscopical 

 Journal," p. 27, October, 1858, On the Optical Powers of 

 the Microscope," by Mr. Rylands. 



P. 61, note.] With regard to the value of the Diato- 

 macece as test-objects, pointed out at page 57, it is only 

 due to Mr. J. D. Sollitt, of Hull, to state that he was the 

 first observer who proposed the use of their shells for 

 test-objects, and published a paper on them in the " Magazine 

 of Science," April 4th, 1846. In 1841 Mr. Sollitt and 

 Mr. Harrison first made out the markings of Pleurosigma 

 attenuatum, not hippocampus, as stated at page 313. The 

 following are the measurements of the cross-lines, in a few 

 of the species : 



Amphipleura Pellucida, or Acus, 130,000 in the inch, cross lines. 



Sigmoidea, 70, 000 in the inch. 

 Navicula Rhomboides, 111,000 in the inch, cross lines. 

 Pleurosigma Fasciola, fine shell, 86,000 in the inch, cross lines. 



strong shell, 64,000 in the inch, cross lines. 



Strigosum, 72,000 in the inch, diagonal lines. 



Angulatum, 51,000 in the inch, diagonal lines. 



Quadratum, 50,000 in the inch, diagonal lines. 



Spencerii, 50,000 in the inch, cross lines. 



Attenuatum, 42,000 in the inch, cross lines. 



Balticum, 40,000 in the inch, cross lines. 



Formosum, 32,000 in the inch, diagonal lines. 



Strigilis, 30,000 in the inch, cross lines. 



Mr. Sollitt says : " I bring out the difficult markings 

 on all these shells in the way described by me in the 

 ' Quarterly Journal of Microscopical Science/ vol. iii. 

 p. 87. No other mode of illumination brings out the 

 lines on the Acus so well. The object-glass shows the 

 Fasciola in squares, and the ^ brings out separate dots." 



