112 THE MICROSCOPE 



is a means of overcoming what would otherwise be a serious 

 error. It is corrected in the Beck binocular by combining 

 a parallel plate of glass of the required thickness with the 

 right-hand prism, thus equality in the focus and in the magnify- 

 ing power of the two images is ensured. The binocular prism is 

 carried in a sliding box in the body of the microscope (Fig. 102). 

 By sliding it out of the optic axis the microscope is converted 

 into a monooidar instrument, or by unscrewing the knob (P, 

 Fig. 102) it can be slid completely out of the microscope for 

 cleaning or dusting. It is quite safe to remove the prism complete 

 in its box, as it returns with accuracy to its exact position, and 

 the adjtistment will not be interfered with. Dust may be removed 

 from the prism with a camel's-hair brush or it may be carefully 

 wiped with a silk handkerchief or leather ; but glass should never 

 be touched with the fingers, a greasy smear damages the defini- 

 tion more than a considerable amount of dust. 



The fact that when the prism moves to one side the instrument 

 becomes absolutely the same as a monocular microscope renders 

 this microscope equally useful for photography, drawing, mi- 

 crometry, or any other purpose. 



Many believe that eventually the binocular wiU be almost 

 universally used, but we recognise that at present this opinion 

 may not be shared by all, and that an opportunity of using 

 either monocular or binocular should be provided. 

 Short The construction of this binocular renders it possible to retain 



le^th. *^6 short tube of the compact monocular microscope. This 

 binocular body, indeed, can be fitted to most of the various 

 recent models of monocular microscopes. When the drawtubes 

 are partially extended, the tube is of the standard 160 Trim, 

 length, the binocular microscope is thus rendered as compact 

 and serviceable as the monocular type. In the older types of 

 binocular microscopes a tube of about 9 to 10 inches in length 

 was required in order to extend the eyepieces to the necessary 

 interocular distance, but examination of the diagram (Fig. 102) 

 shows that, owing to the peculiar construction of the prism, the 

 tubes, instead of converging towards the prism, converge to 

 an apex about 3 J inches below it ; thus, although the standard 

 angle of normal convergence is retained, the tubes need not 

 be long to give the required separation for the eyes. The 

 tubes converge at an angle of about 14°. This wiU be found 

 in practice to give absolute comfort for either long or short 

 periods of working. The eyes are in exactly the condition 

 required for reading a book. 



Binocular telescopes which are used with the eyes looking out 

 horizontally at distant objects generally and correctly have their 

 two tubes parallel, but tMs is unsuitable for a microscope. The 

 microscopist who uses his instrument alternately with examining 

 objects on the table on which it stands would find it difficult 



