THE MICROSCOPE STAND 107 



large 2-incli diameter, with a diawtube giving a variation in 

 length from 140 mm. to 200 mm. A rack and pinion double 

 extension drawtube, as illustrated on page 103, can be fitted if 

 desired. An interchangeable binocular body can be fitted to 

 the instrument, and a circular rotating stage can be made in 

 place of the square stage ; but in this case a gap cannot be cut 

 out in front, and some of the advantages of the interchangeable 

 substage apparatus are lost. The apparatus can be interchanged, 

 but only after racking down the substage by the amount of the 

 thickness of the stage. 



With .'this massively made microscope, the body and apparatus 

 can be relied upon to be always truly in the optic axis, the 

 manipulation of one part of the instrument does not tend to 

 upset the adjustment of the other parts, and when using the very 

 highest power lenses it is a pleasure to work on account of its 

 stability and the delicacy of all its adjustments. 



The Binocular Microscope 



Hitherto binocular microscopes have not been used for 

 research work, except in special cases. For practical purposes 

 the monocular has for many years held the field, and the use of 

 binoculars has practically been restricted to workers who only 

 use low powers, or for exhibition purposes. The reason of this 

 ip simply explained : the one advantage of using two eyes did not 

 outweigh the loss of the many advantages possessed only by the 

 monocular stands. 



Binocular microscopes may be divided into three types, and 

 a brief description of each type will set forth their 

 respective merits and demerits. 



Type 1, best represented by the " Wenham," 

 bisected the beam of light that emerged from the 

 object glass (0, Fig. 99) and directed the right-hand 

 half into the one eye, the left-hand half into the 

 other. 



Binocular vision with this was not equal to 

 monocular, because by reducing the size of the B'la. 99. 

 beam of light which formed each image it reduced 

 the resolving power of the microscope. It could not be used 

 with high-power object glasses, because the prism could not be 

 placed sufficiently close to the back lens of the object glass to 

 properly bisect the beam of light into two separate halves 

 before the rays had intermingled. Efforts to accomplish this 

 by mounting high-power object glasses in special short mounts 

 only partly overcame the difficulty, and rendered the use of 

 revolving nosepieces impossible. 



This type of instrument involved long tubes and con- 

 sequently bulky instruments, and could only be satisfactorily 



