THE mOROSOOPB STAND 115 



and therefore such suggested diapliragms cannot be placed in the 

 correct positions — in fact, due to the eyelids and eyelashes of the 

 observer, they cannot even be placed near the correct position. 



But there is another method of stopping out the portions 

 required to give a stereoscopic picture. 

 If the eyepieces be placed at a slightly 

 incorrect interocular distance, the pupils 

 of the observer's eyes cut ofi the edges of 

 the two Ramsden discs (Fig. 107), and 

 as the stereoscopic effect with a high- 

 power object glass is generally exag- 

 gerated, a very small movement is 

 sufficient to give perfect depth of vision. 

 The tubes of the new microscope 

 are, however, inclined, and there is no 

 necessity to vary the interocular dis- 

 tance. The observer naturally places 

 his eyes so that the whole of the Ramsden Pi**- 



discs (Fig. 107) enter the pupils of the eyes, 



and obtains all the advantages as to aperture, resolution, and illu- 

 mination of a monocular microscope. Then, by moving his head 

 either forward or backward, h^ cuts ofi with his pupils the one 

 or other side of the Ramsden discs and obtains either stereoscopic 

 or pseudoscopic relief instantly. The movement required is scarcely 

 over an eighth of an inch, and the result is that aUthe advantages 

 of stereoscopic relief are obtained without sacrificing anything. 



The result of the movement of the head is very astonishing : 

 if objects are being examined which lie on difierent levels, one 

 point appears either in front of or behind another at will, and the 

 position of the observer's head indicates which is the stereoscopic 

 or pseudoscopic picture. 



The Beck high-power binocular body can be supplied on any 

 of the microscopes illustrated, either in place of the ordinary 

 body or as an extra interchangeable body. 



MetaEurgical microscopes require certain special features Metai- 

 because almost all objects for which they are used require j2['^^Jopgg_ 

 illumination from above. A great deal of their examination 

 is done with high powers with one or other of the vertical illu- 

 minators mentioned on page 41. It is, therefore, important that 

 the beam of light for the use of these illuminators, having once 

 been adjusted, should be allowed to remain in a fixed position. 

 If the body tube of the microscope to which these illuminators 

 are attached is focussed up and down to examine specimens of 

 different thickness or to enable different object glasses to be used, 

 the illuminator cannot be kept opposite to the illuminating beam 

 of light. Metallurgical microscopes must, therefore, be made in 

 a manner that will overcome this difficulty. The three following 

 forms of microscopes show three methods of accomplishing this. 



