COMPOUND MICROSCOPES. 349 



lined with a simple magnifier. The object to be viewed is brightly 

 illuminated, if solid, by condensing upon it the light of a lamp by 

 means of a subsidiary lens, or reflecting light on to it by a concave 

 mirror, or both ; if transparent, by similarly passing through it a 

 condensed beam of light ; the object glass being placed so near to 

 the object that the distance between is a little greater than the 

 focal length of the object glass, a real inverted magnified image is 

 formed on the other side of the lens and further off precisely as in 

 a magic lantern. This is then viewed through a magnifying eye 

 piece after much the same fashion as the image formed by the 

 object glass of an astronomical telescope. When the object glass is 

 of short focal length ( J-inch, J-inch, or less), the real image formed 

 thereby at several inches distance on the other is largely magnified, 

 and this magnification is multiplied again by the eyepiece. When 

 " high powers " are used (object glasses of small focal length and 

 highly magnifying eyepieces), the lenses and combinations of 

 lenses used must be carefully proportioned and regulated to avoid 

 chromatic aberration and similar sources of imperfect definition. 



Fig. 192 represents a simple form of compound microscope, 

 where the object to be viewed is placed on the stage, and illumi- 

 nated from below by light reflected from a mirror. 



Microscopes are sometimes fitted as a photographic camera at 

 the eyepiece end, so that instead of employing the eye to observe 

 directly, a small photographic glass slide, &c., is placed in position, 

 and a highly magnified real image of the object viewed focussed 

 on the slide, so as to obtain a permanent photograph of the object. 

 Telescopes are also sometimes used in the same way, eg., in obtain- 

 ing photographs of nebulae, &c. ; in this case the telescope must be 

 moved by a clockwork arrangement, so as to keep it constantly 

 pointing in the same direction; otherwise the rotation of the earth 

 would cause the position of the image of a given object to be 

 always in motion, which would prevent a clearly defined photo- 

 graph being obtained with any objects emitting but little light, 

 and therefore requiring a considerable time of exposure (vide 

 Chapter XXV.). 



