1 82 HOW TO WORK 



piece of glass. The upper surface should ihen be filed level, and 

 dressed with coarser and finer emery paper, and afterwards ground 

 smooth with a bit of a soft Water-of-Ayr stone about -inch 

 square. Every trace of roughness should then be removed by 

 means of rouge and water on cloth ; for unless the surface be 

 extremely well polished the structure of some kinds of iron cannot 

 be seen. It must not be that sort of polish which merely gives 

 a bright reflection, but one which may show all the irregularities 

 of the material, and is as far removed as possible from a burnished 

 surface. All trace of the rouge should be washed off, and care 

 used not to touch the surface with the fingers, which is then acted on 

 with extremely dilute nitric acid. If the action be allowed to pro- 

 ceed too far, the most important points in the structure may be 

 entirely obliterated ; and therefore it is well to take the section out 

 of the solution and examine it under water in a glass trough, and 

 again act on it wkh acid, time after time, until the structure is seen 

 to the greatest advantage. The section must then be well washed 

 and quickly dried by wiping the surface with a handkerchief; and 

 after slightly nibbing it on soft wash-leather, a thin glass cover must 

 be mounted over it with Canada balsam. 



Of course such sections must be examined by reflected light 

 For this purpose no illuminator is better than the parabolic reflec- 

 tors supplied by Messrs. Beck, which were in fact first made for 

 me, for that special purpose. I afterwards added another small 

 reflector, inclined at an angle of 45, attached to a moveable arm, so 

 that we may see an object by direct reflection. The general con- 

 struction will be seen from fig. 265, pi. XLII, copied from 

 Mr. Richard Beck's paper in the Transactions of the Microscopical 

 Society (vol. XIII, p. 117). 



The small reflector is seen at tn, with a semi-cylindrical tube _r, 

 to shut off the light reflected by the parabola. When the latter 

 is used the small reflector is turned away by means of the milled 

 head w, into the position indicated by the dotted lines. The differ- 

 ence between the two illuminations will be seen from figs. 266 and 

 267. When the parabola is used, light passing from d is reflected 

 from f, fig. 266, and if the object b has a polished surface, is again 

 reflected to e, quite outside the object-glass a, so that a polished 

 surface appears black, whilst at the same time a rough surface 

 appears white or coloured by diffused reflection. When, how- 

 ever, the small reflector g, fig. 267, is half over the object-glass, 

 the light is reflected through the other half of the lens c, in such a 

 manner that a polished surface appears bright, and a rough surface 

 comparatively dark. We can thus distinguish at once the difference 



