292 White Cloud Illumination. 



Fig. 15. Graptolithus priodon. Section magnified. 

 Fig. 16. Graptolithus Roemeri. ditto. 



Fig. 17. Rastrites, sp. Fragment magnified. 

 Fig. 18. Diplograpsus pristis. Section magnified. 



{To be concluded in an early number.) 



A WHITE CLOUD ILLUMINATION FOR LOW 

 POWERS. 



BY HENKY J. SLACK, F.S.A., HON. SEC. R.M.S. 



Microscopists have long adopted various modes of obtaining 

 what is technically known as a " white cloud illumination/' or 

 illumination resembling that of solar light reflected from a 

 white cloud. Some years ago a favourite plan was the employ- 

 ment of a disc of plaster of Paris in the place of the flat 

 mirror ; but such discs soon get dirty, and are therefore incon- 

 venient. The white porcelain glass shades of lamps, like Mr. 

 Pillischer's, enable an effect of this kind to be obtained, by so 

 arranging a bull's-eye condenser, or stage mirror, that no 

 direct light from the lamp-flame reaches the object, but only 

 such rays as have previously suffered reflection from the por- 

 celanous surface. 



Another method employed with advantage, and recom- 

 mended many years ago by the writer, was to take a disc of 

 white foreign post paper, place it on a stouter paper, cover it 

 with a few little bits of spermaceti and hold it over a lamp till 

 the thin paper was thoroughly saturated with the molten 

 matter. Such a piece of paper can be inserted in the frame 

 carrying a bulPs-eye, next its flat surface, or may be placed in 

 the diaphragm hole under the stage. A still better plan for all 

 but very low powers is to cut a small square or disk of the thin 

 paper about seven-eighths of an inch in diameter, place it on an 

 ordinary slide, in the middle, saturate it with spermaceti, and 

 while the latter is hot place over it a thin covering glass, of a 

 trifle larger size. This keeps the spermaceti always clean. 

 The way to use it is to place the spermaceti slide on the- stage, 

 and the slide, carrying the object to be viewed, upon it. * This 

 gives a very pleasing illumination, and answers for all powers 

 sufficiently high not to focus through the object- slide and 

 show the surface of the spermaceti paper below it. 



When using a binocular instrument, with powers of 1-J. to 

 3 inches, ordinary methods fail to produce the soft, luminous, 

 and opaque-looking background which is best fitted for large 



