Under the effects of sunlight the pigment granules are gathered together into small rounded spots, as seen on the left hand of the figure, leaving the skin of its own bright yellow hue. In the shade the pigment granules spread themselves so as to cover almost the entire skin and to produce the dark brown colour. In the intermediate state they assume the bold stellate form in which they are shown on the right hand of the round spots. Very remarkable forms of these cells may be found in the skin of the cuttle-fish.
Figs. [24] and 25 are two examples of coal, the former being a longitudinal and the latter a transverse section, given in order to show its woody character. Fig. [17] is a specimen of gold-dust intermixed with crystals of quartz sand, brought from Australia; and Fig. [21] is a small piece of copper-ore.
Every possessor of a microscope should, as soon as he can afford it, add to his instrument the beautiful apparatus for polarising light. The optical explanation of this phenomenon is far too abstruse for these pages, but the practical application of the apparatus is very simple. It consists of two prisms, one of which, called the polariser, is fastened by a catch just below the stage; and the other, called an analyser, is placed above the eye-piece. In order to aid those bodies whose polarising powers are but weak, a thin plate of selenite is generally placed on the stage immediately below the object. The colours exhibited by this instrument are gorgeous in the extreme, as may be seen by Plate [XI]., which affords a most feeble representation of the glowing tints exhibited by the objects there depicted. The value of the polariser is very great, as it often enables observers to distinguish, by means of their different polarising properties, one class of objects from another.
If the expense of a polarising apparatus be too great for the means of the microscopist, he may manufacture a substitute for it by taking several thin plates of glass, arranging them in a paper tube so that the light may meet the surface of the lowest one at an angle of about 52°, and placing the bundle above the eye-piece to act as an analyser; whilst, by using a plate of glass, and so arranging the lamp that the light falls upon it at the above angle, and is reflected up the tube of the microscope, he will find on rotating the extemporised analyser that the phenomena of polarisation are to a great extent reproduced; whilst by splitting an extremely thin film from the surface of a sheet of mica, such as is employed for making smoke-screens above glass globes, he will have a substitute for the selenite by means of which alone can the gorgeous effects be produced. The extemporised apparatus will not, of course, give such perfect effects, but this is sometimes an advantage, and the present writer has used the same means with considerable success in photographing starch-granules.
XIV.
| FIG. | |||
| 1. | Dero latissima | 13. | Brachionus amphiceros |
| 2. | Slavina serpentina | 14. | Salpina mucronata |
| 3. | Pristina longiseta | 15. | Pterodina patina |
| 4. | Lophopus crystallinus | 16. | Anurœa brevispina |
| 5. | Fredericella sultana | 17. | Dinocharis tetractis |
| 6. | Stylaria proboscidea (head) | 18. | Noteus quadricornis |
| 7. | Hydatina senta | 19. | Floscularia ornata |
| 8. | Synchœta mordax | 20. | Young Melicerta |
| 9. | Asplanchna Brightwellii | 21. | Macrobiotus (sp.?) |
| 10. | Notommata aurita | 22. | Cypris fusca |
| 11. | Triarthra longiseta | 23. | Cyclops quadricornis |
| 12. | Euchlanis triquetra |
XIV.