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 2. Slavina serpentina 3. Pristina longiseta 4. Lophopus crystallinus 5. Fredericella sultana 6. Stylaria proboscidea (head) 7. Hydatina senta 8. Synchœta mordax 9. Asplanchna Brightwellii 10. Notommata aurita 11. Triarthra longiseta 12. Euchlanis triquetra 13. Brachionus amphiceros 14. Salpina mucronata 15. Pterodina patina 16. Anurœa brevispina 17. Dinocharis tetractis 18. Noteus quadricornis 19. Floscularia ornata 20. Young Melicerta 21. Macrobiotus (sp.?) 22. Cypris fusca 23. Cyclops quadricornis [Illustration: XIV.] CHAPTER XI Hints on the Preparation of Objects--Preservative Fluids--Mounting Media--Treatment of Special Objects.