156 W. M. Hutchings — Ash-slates of the Lake-District. 



It is very difficult to make out much as to its exact nature under 

 the microscope ; nothing but the very thinnest films are of any use. 

 Taking the thinnest edges of thin sections, or the margins of little 

 holes worn in the last stages of grinding, and studying these 

 carefully with a -^ or ^ objective and good illumination,^ the most 

 prominent constituent is seen to be a very pale-green chlorite in a 

 felted mass of extremely minute flakelets, mainly arranged flat in 

 the plane of cleavage. In polarized light one sees large numbers of 

 minute rod-like bodies lying in all directions in among the chlorite, 

 strongly bi-refractive, extinguishing quite parallel, their long axes 

 being axes of least elasticity. In ordinary light these rods are not 

 -Seen at all. There are also countless minute grains of another minei-al, 

 mainly so small that, owing to the very high refraction which they 

 possess, they appear as opaque bodies ; but larger ones occur with 

 transpai'ent colourless to yellow centres. These grains are so 

 numerous that the whole slide, under low power, seems simply full 

 of a dark dust. The largest grains attain to a diameter of about 

 i2-oo^o- inch. By examining such of these larger grains as occur at 

 spots in the slide which remain dark on rotation between crossed 

 Nicols, it is seen that they are isotropic. 



On rotating the section in polarized light, the whole appears as 

 a faintly-speckly, cryptocrystalline, felsitic-looking mass, full of the 

 brightly-polarizing minute rods above spoken of, and one suspects 

 that there is some other matter present besides those mentioned, 

 but it is so wholly enveloped in and masked by the chlorite that 

 nothing can be safely made out. 



If a thin section is treated alternately with hot hydrochloric acid 

 and solution of caustic potash, the chlorite may be mostly removed ; 

 but this operation succeeds better using powdered slate, digesting 

 thoroughly with moderately strong acid, washing free from the dis- 

 solved chlorides and treating with hot potash solution to remove 

 separated silica, afterwards mounting the small grains and flakes in 

 balsam. Material so prepai'ed is free from chlorite and reveals 



' In the matter of illumination, I have for a long time given up attempting to 

 work by daylight, which in this climate is usually of too interior a quality and too 

 capricious for anything but low -power work. After experimenting in various ways 

 I now make use of an ordinary good microscope-lamp burning paraffin. The lamp- 

 chimney is of pale-blue glass, and the buH's-ej'e condenser used with it is also blued, 

 by cementing to the fiat side of it, with Canada-balsam, sufficient thicknesses of 

 pale-blue glass so that the field of light obtained in the microscope appears bright 

 white, like the best daylight. The adjustment of the quality of the light can also 

 be controlled by observing the colours in polarized light of several points on a 

 quartz-wedge (or better, on a mica-wedge made as recommended by Mr. Dick) first 

 in good daylight and then in the light of the lamp. The depth of blue glass added 

 to the condenser should be such that the colours are exactly the same in both cases. 

 Then, with a good full flame and using the broad side of it, the light should be so 

 thrown on to the mirror, from the flat side of the condenser, that the mirror is 

 just covered with light. The effect in the microscope is the same as working with 

 an exceptionally fine sky and one is sure that one's work will not be spoilt by a 

 change in the light. Anybody who has once got used to working in this way and to 

 the great advantage of always working with the same light will not again forego its 

 advantages, but will, even in the daytime, make exclusive use of artifical light. 

 My own experience also is that work with a good steady %vhite light of this sort is 

 less trying to the eyes than anything else. 



