ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 731 



" The tube of the Microscope must be tested each time before it is 

 used to see whether any foreign body is enclosed in it, or one of the 

 diaphragm stops has got on edge. The draw-tube must be pulled 

 out before use. The glasses of the lens-systems of the instrument, 

 and also the illuminating mirror, are to be carefully cleaned with a 

 dry hair-pencil, or with very soft wash-leather. 



With illumination by light from below, care must always be taken 

 that it falls as horizontally as possible on the mirror. The Microscope 

 should therefore not be brought nearer to the window than is abso- 

 lutely necessary. Dazzling sunlight is a disadvantage. Double 

 windows are an impediment to the examination. 



Only in exceptional cases are examinations to be made by lamp- 

 light, and on such occasions a low petroleum lamp is to be used, with 

 a glass shade, the lower part of which is closed either by porcelain- 

 glass or ground white glass. 



Those who desire to examine with low powers and light from 

 above must bring the Microscope near the window in order to obtain 

 as much incident light as possible. 



The hours of bright daylight are to be chosen for the examination, 

 and the work should be done, if practicable, at an open window. 



The greatest care must be used in attaching to the tube the 

 systems selected, and the operator must make sure that the tube is 

 exactly centered. Particular attention nrast be given to the estimation 

 of the focal distance. With low powers the focal distances are much 

 greater than with high-power objectives, and the tube will therefore 

 require a greater distance between it and the preparation, in propor- 

 tion as the powers used are low. 



The preparation to be examined is now placed, with the cover-glass 

 on, in such a position on the stage that it lies as nearly as possible 

 over the centre of the opening of the stage. The largest diaphragm 

 aperture is then to be brought underneath, and full direct light 

 reflected by the mirror up the tube. Whilst the eye, kept as near as 

 possible to the eye-piece, is directed towards the object, the tube is 

 cautiously moved up and down till the image appears clearly." 



The Prefect of the Seine has also recently established a course of 

 six lectures for the teaching of micrography, and an examination has 

 been instituted for inspectors for detecting Trichinae in the substance 

 of pork and ham of American or German origin imported into France, 



Continuous Observations of Minute Animalcula.* — E. Holmes 

 having found some difficulty in keeping minute living objects under 

 observation on account of the water evaporating, and also that any 

 attempt at a supply produced currents which washed away all very 

 small organisms, was led to put upon a slide a small quantity of 

 water, and a very minute portion of plant, not using enough water to 

 occupy all the space under the cover-glass, but leaving part of it occu- 

 pied by air. A ring of paraffin wax was put round the cover, thus 

 sealing up the contents, embracing rotifers and diatoms, several 

 hundred species in all. At the expiration of a week they were still 



* Sci.-Gossip, 1882, p. 138. 



