122 The Microi^cope. 



-careful focussing and illumination. 2. The inner layer is nearly 

 flat and consists of " a square grating set lengthwise of the valve," 

 the alternate squares l>eing red, with the intervening ones white, 

 the white becoming green and the red white with deeper focuss- 

 ing. 3. The middle layer is also a grating of bars of silex crossing 

 each other nearly at right angles, in other words a plate with 

 approximately square perforations, the grating set obliquely to 

 the length of the valve. 4. The other layer is made up of fibrils 

 running the whole length of the valve, each pair separating and 

 meeting, alternately, so as to pass round each dot in the middle 

 grating, and when separated from the shell, the fibril appears 

 like rectangular blocks of silex attached to each only by the 

 •corners. 



Mr Cox presents the present state of our knowledge with re- 

 gard to microscopical vision and the interpretation of microscopi- 

 cal images. He also sums up the accepted ideas as of the struct- 

 ure of the Diatom shell, showing that the new points presented 

 in Mr Smith's paper are: 1, the asserted existence of an inner 

 tessellated plate having squares showing alternate colors under 

 the microscope, and twice as numerous as those in the middle 

 layer lying immediately above it ; and 2, the outer coat of fibrils 

 (silicified) running so as to leave openings above the " dots '" or 

 .alveoli of the principal plate. 



As to the first of these, Mr C"ox argues that the known phe- 

 nomena of diffraction and oi microscopical definition will per- 

 fectly account for the appearance of Mr Smith's inner tessellated 

 plate, the red squares being caused b}' the solid area of silex in 

 the interspaces between the alveoli, the latter being approximately 

 round, arranged in lines nearly at right angles to each otiier. 

 The green or white spots he regards as the true alveoli. 



As to the fibrils, however, Mr Cox thinks that the photographs 

 sent by Mr Smitli demonstrate their presence in the specimens 

 from which the photographs were taken, the decisive fact being 

 that some of them have been floated off" and lie beside the shell 

 which lias similar ones in situ. Whether this is a peculiar state 

 of the specimen which might be seen with ordinary objectives, 

 ■or is a revelation due entirely to the apochromatic lenses, Mr Cox 

 thinks deservin*-; of further examination. 



