WHITB-CLOUD AND OBLIQUE ILLUMINATORS. 135 



ently attaclied by a jointed arm to- the frame whicli carries the 

 disk, according to the method of Messrs. Powell and Lealand, 

 shown in Fig. 38 ; the frame itself being made to fit upon the 

 mirror, and to turn with it in every direction. Another very 

 simple, and for many purposes very effi- fio. 38. 



cient mode of obtaining a white-cloud 

 illumination (invented by Mr. Handford) 

 consists in coating the back of a concave 

 plate of glass, like that employed in the 

 ordinary concave mirror, with white zinc 

 paint, instead of silvering it; and then 

 mounting this in a frame, which may be 

 fitted_(like the plaster of Paris disk just white-cioud illuminator. 

 described) over the ordinary mirror. A concave surface of plas- 

 ter of Paris, moreover, might easily be obtained by casting it 

 when fluid upon the convex surface of such a plate. When a 

 concavity is thus given to the white surface, its performance with 

 low powers is much improved ; but with high powers, a special 

 condensation of the light must be adopted, and the arrangement 

 above described seems the simplest that could be devised. It is 

 open, however, to certain objections, which become apparent 

 when very high powers are used and difficult objects are under 

 examination ; and to obtain the most perfect white-cloud illumi- 

 nation possible, is the object of the apparatus devised by Mr. 

 Gillett. This consists of a small camphine lamp, placed nearly 

 in the focus of a parabolic speculum, which reflects the rays 

 either at once upon a disk of roughened enamel or upon a second 

 (hyperbolic) speculum which reflects them upon such a disk. A 

 very pure and concentrated light is thus obtained ; and as the 

 forms of the incident pencils are broken up by the roughened 

 surface, that surface takes the place of the lamp, as the source 

 from which the rays primarily issue. The advantage of this illu- 

 mination is specially felt, in the examination of objects of the 

 most difficult class under the highest powers. 



59. Oblique Illuminators. — ^It is frequently desirable to obtain 

 a means of illuminating transparent objects with rays of more 

 obliquity than can be reflected to .them from the mirror, even 

 when this is thrown as much as its mounting will permit out of 

 the axis of the Microscope (§ 89), or than can be transmitted by 

 the ordinary achromatic condenser, even when all but its mar- 

 ginal aperture is stopped out. Such oblique light may be used 

 in two entirely different modes. The rays, although very far out 

 of the axis of the microscope, may still not make too great an 

 angle with it to fall beyond the aperture of the objective ; and 

 thus, entering its peripheral portion after their passage through 

 the object, they will form the image in the ordinary way. The 

 advantage of such oblique illumination, arises from its power of 

 bringing out markings which cannot be seen when only direct 

 rays are employed ; and when the rays come only from one side, 



