246 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



To obtain images in relief, which cannot take place when observed 

 with one eye only, binocular microscopes are now constructed. In 

 Cachet's arrangement (Fig. 187), the image formed by the object- 

 glass is divided into two portions; which are reflected to opposite 

 sides, and again reflected up two parallel tubes, placed at the width 

 of the eyes, each having an eye-piece. In the form introduced by Mr. 

 Wenham (Fig. 186) the image is divided into two parts by a prism, 

 which by a double reflection bends one at an angle and throws it up one 

 tube whilst the other part passes direct up the other tube. Another 

 system, introduced by Mr. Stephenson, has the advantage of giving 



far better results with high magni- 

 fying powers. The necessity of 

 this arrangement will be understood 

 when we study stereoscopic vision. 



Lastly, special microscopes are 

 made (Fig. 182) in which the eye-- 

 piece tube is inclined, and terminates 

 under the stage. A prism sends the 

 luminous rays in the direction of 

 the eye by total reflection. These 

 instruments are made for chemists 

 to examine objects through the glass 

 bottom of a small vessel containing 

 a liquid. 



Within the last dozen years 

 spectrum analysis has been applied 

 to microscopical research by Mr. 

 Sorby, to study accurately the exact 

 nature of the light transmitted by, 

 or reflected from, minute coloured objects. This is usually accom- 

 plished by means of a special eye-piece, containing a slit and 

 compound direct-vision prisms, and an arrangement so that the 

 spectrum of another object on a side stage may be compared with 

 that of a smaller object magnified by the object-glass. By another 

 plan the spectrum apparatus is placed under a special object-glass of 

 long focus, so that it can be used with a binocular microscope and 

 the spectrum seen with both eyes. 



We shall terminate this description of microscopes by mentioning 



FIG. 187 Nachet's binocular microscope. 



