44 PRACTICAL MICROSCOPY. 



be used with the 2-inch ocular (the A eye-piece), the 

 former will produce an enlarged image of 10 diameters, 

 which the ocular will again magnify five times, producing a 

 total amplification of fifty. 



This is not mathematically exact without every disturb- 

 ing element be taken into consideration, but is quite near 

 enough to illustrate the case in practice. Fifty diameters 

 is the recognised amplification for the i-inch objective 

 combined with the A ocular at a distance of 10 inches, and 

 as the enlargement of the object takes place in two distinct 

 stages, it will be seen that the optician is able to vary the 

 powers of both ocular and objective and still obtain the 

 standard result. An inch (so-called) objective magnifying 

 8 3 diameters when used with an A eye-piece magnifying 

 the image six diameters, will give a normal result, as will 

 an objective of the same designation magnifying 12 '5 

 diameters with an A ocular magnifying four. 



Similar cases to both of these have recently fallen under 

 the notice of the author, and it is on account of like depar- 

 tures, that abnormal amplifications are obtained when the ob- 

 jectives of one maker are used with the oculars of another. 



It often happens with new oculars that particles of brass 

 get detached, and fall upon the inner surface of the lenses, 

 which must be removed by unscrewing, and then carefully 

 wiping with a very soft wash-leather. Dust specks and 

 bubbles may be easily detected by deflecting a dull light 

 through the body of the instrument, when, by observation 

 during the rotation of the eye-piece, they show very plainly. 

 A good eye-piece should be perfectly transparent, and free 

 from striae and markings and spots of any kind. The 

 marginal circle of the field of vision should be sharp, clear, 

 and intensely black. If these conditions are not fulfilled, 

 the eye-piece cannot be considered as perfect, or fit for 

 general use. 



