UNIVERSAL REFLECTING ILLUMINATOR. 217 



extreme obliquity of the light, let the reflector be placed as far back 

 as possible, or from an inch to an inch and a quarter or more, 

 according to the power to be used, so as to produce a dark-ground 

 illumination, (to effect this every different object-glass will require a 

 different angle, according to its angular aperture and its working 

 distance from the object,) and when this has been obtained, let the 

 speculum be gradually advanced until the markings become suffi- 

 ciently distinct, when the light may be finally adjusted for intensity, 

 &c. Should the preparation, however, be one only requiring a 

 moderate degree of obliquity, such as, for instance, injections, or 

 parts of insects, or similar objects, let the frame (F) be drawn 

 forward, until the slit shall be nearly in a line with the optical axis, 

 when the reflector may be inclined, as in Fig. 3, and then be moved 

 back until a satisfactory effect shall have been produced. It may be 

 observed that, with preparations that are translucent rather than 

 transparent, the light may be thrown at such an angle as to be re- 

 flected down upon the object from the under side of the glass cover, 

 and thus, by the combination of transmitted and reflected rays, 

 render its detail remarkably clear and distinct. The bull's-eye lens, 

 which is an inch and a half in diameter, and about two inches focus, 

 requires to be attached below the stage in a corresponding position 

 to that which it usually occupies above, only, in this case, each joint 

 needs to have a tightening screw to prevent its settling down by its 

 own weight. 



There are some objects, however, that are best seen under an 

 illumination intermediate between a dark ground and a bright field, 

 and which will be easily found by advancing or receding the re- 

 flector. With the low powers and with daylight the concave mirror 

 may be used by placing it centrally and inclining the reflector in a 

 slight degree forwards, and moving it until the whole of the field 

 shall have become illuminated. Where stereoscopic effect is desired, 

 it will be found to be very materially affected by the inclination of 

 the light, and when this has been arranged at its best, it will be 

 found to contrast highly favourably against the ordinary modes of 

 illumination. 



If the reflector be made of very white silver, or speculum metal, 

 it will afford a light of extreme brilliancy ; but this dazzling white- 

 ness is highly objectionable for two very important reasons ; in the 

 first place, under it, objects are not seen best, and next, it is 

 extremely trying and injurious to the eyesight. The tint of the 



