52 The Illumination of Objects. 



denser, and gives an illumination from a high, degree of 

 obliquity to one consisting of a small pencil of nearly parallel 

 rays. Mr. Highley is at present engaged in devising improve- 

 ments in this instrument, and we shall take an early oppor- 

 tunity of describing its most finished form, merely saying at 

 present that it promises to be of great service to micro scopists, 

 from its utility and comparatively small cost. 



Various experimenters have tried the effects of monochro- 

 matic light. We do not know who began such trials. We 

 commenced them several years ago by affixing flat discs of 

 coloured glass on the flat side of an ordinary bull's-eye con- 

 denser. Probably very small pencils of light would be most 

 successful, and Mr. Collins' " Graduating Diaphragm " would 

 facilitate such experiments. Red light, as least refrangible, 

 and violet light, as most so, would give the most conspicuous 

 results ; but there are occasions when objects are well seen on 

 a coloured background, and we remember Messrs. Powell and 

 Lealand making an interesting exhibition of this kind. 



In our last number (page 481) we mentioned that the Abbe 

 Count Castracane employed — as is stated, with success on 

 diatoms — Foucault's heliostat and a prism of large dispersion. 

 For the Pleurosigma angulatwm he found a blueish green 

 light the best. As a rule, we should distrust the effects 

 produced in the glare of sunlight illumination, but there are 

 occasions when some observers consider it may be used with 

 advantage. 



In illuminating opaque objects the direction of the light is 

 highly important. A pleasing instance of this occurs in the 

 display of the mineral called hyperstene. A ray of nearly per- 

 pendicular light makes this object look as unlovely as a piece 

 of coal, while one of exactly the right slope brings out gorgeous 

 colours, often arranged like a series of Chinese pictures of 

 landscapes and rivers. Mr. Sorby found in some of his impor- 

 tant researches that the structure of certain objects could only 

 be made out by nearly vertical illumination, and Messrs. Smith 

 and Beck constructed some apparatus adapted to this want. 

 For ordinary purposes a side silver reflector, which is best 

 mounted on a separate stand, is an admirable illuminator, as it 

 enables the operator to transmit rays of varying obliquity, 

 though none as vertical as in the arrangement just described. 

 There has been a tendency of late years to undervalue the 

 Heberkuhn, but this is a mistake. It can be used well with 

 object-glasses from one inch to a quarter, if not focussing too 

 close, and by varying the position of the mirror the light may 

 be condensed more strongly on one side than on the other. It 

 would probably be worth while trying the experiment of limit- 

 ing the size of tho pencil of light reaching a Heberkuhn by 



