xvi EXPLANATION OF PLATES 



PLATE XI. (Facing p. 594) 



This plate has a twofold purpose. It is designed, first, to justify the 

 opinions held by Dr. Henry varn Heurck upon the structure of the valves of 

 diatoms, and also to show how the usual microscopical tests present them- 

 selves when examined with the new objective with N.A. 1-60, lately constructed 

 by the firm of Zeiss. This objective is believed by Dr. van Heurck to realise 

 what he considers the highest results of photographic optics, which in his 

 judgment could only be surpassed by finding a new immersion liquid of still 

 higher refractive index presenting all the necessary qualities, and which at the 

 same time would not affect the very delicate flint of which it is necessary to 

 make the front lens of this objective. This medium he hopes may be some day 

 realised. Unfortunately, up to this time, no indication permits us to foresee 

 the discovery of the liquid desired. 



The following is the way in which Dr. Henry van Heurck summarises his 

 ideas upon the structure of the valve : 



1. The valve of diatoms ' is formed by two membranes or thin plates and 

 by an intermediate septum. By this he understands a plate pierced with 

 openings. The superior membrane, often very delicate, may be destroyed 

 in the treatment by acids in the washings, by rubbing, &c. It is possible also 

 that it sometimes only exists in a very rudimentary state. The majority of 

 the students of diatoms agree in believing that these membranes may be suf- 

 ficiently permeable to permit of exchange by endosmose between the contents 

 of the valve and the surrounding outer water, but that these membranes have no 

 real openings so long as the diatom is living and intact. 



2. When the openings of the septum are disposed in alternate rows, then 

 they take an hexagonal form. When in perpendicular rows then the openings 

 are square or elongated. The hexagonal form, which is besides so frequent in 

 nature, seems to be the typical form of the openings of the septum, and it 

 is found most frequently when the valve is large, destitute of consolidated 

 sides, and must offer resistance to outside agents. Even in the forms of the 

 square openings we see very frequently deviations and returns to the hexagonal 

 type upon certain parts of the valve. It is possible that the septa may be 

 sometimes composed of many layers, placed one above another, formed succes- 

 sively and closely united ; but up to this time we have no proof of it, neither 

 have we met with any form presenting layers placed one above another. 



Such, in brief, is the view held by Dr. van Heurck as an interpretation of 

 our present knowledge of the structure of the valve of the diatoms. We give 

 now a description of the objects represented on the plate. 



Figs. 1, 2, 3. Amphipleura pellucida, Kiitz, 1 and 2, valve resolved into 

 pearls. Fig. 2 x 2000 diams. Fig. 1 x 3000 diams. Fig. 3. Valve resolved 

 in strice at about 2300 diams. 



Fig. 4. Amphipleura Lindheimeri, Gr., x 2500 diams. 



Fig. 5. Pleurosigma angulatum, in hexagons, x (about) 10,000 diams. 



Fig. 6. Idem x 2000 diams., illusory pearls which are formed by the angles 

 of the hexagonal cells when the focussing is not perfect. 



Fig. 7. The nineteenth band of Nobert's test plate. This photo-micro- 

 graph has been made exceptionally with the apochromatic i O f 1-4 N.A. 

 The lines, being traced upon a cover in crown-glass, the objective of N.A. l-f> 

 cannot be used here. 



Fig. 8. Surirella gemma, Ehrb. x (about) 1000 diams. 



Fig. 9. Van Heurckia crassinervis, Breb. (Frustulia saxonica, Eabh) x 2000 

 diams. 



All the photo-micrographs (except fig. 7) have been done with the new J_- 

 inch N.A. 1-60 of MM. Zeiss. 



These micro-photographs have been produced by sunlight in a monochro- 

 matic form, the special compensating eye-piece 12, and the Abbe condenser of 

 N.A. 1*6. 



1 ' The Structure of the Valve of Diatoms ' in Records of the Belctian Society, 

 v. xiii. 1890. 



