MAGNIFYING POWERS. IfJ 



may be considerably increased without any great regard to cor- 

 rectness of the details. For this purpose, Dr. Carpenter states,* 

 " That he has been in the habit of using an eye-piece consisting 

 of a meniscus, having the concave side next the eye, and a con- 

 vex lens, having the form of least aberration with its flattest 

 side next the object; this form nearly resembles Herschell's 

 aplanatic doublet. The field-glass also is a double convex lens 

 of the form of least aberration. With this eye-piece he was 

 enabled to obtain a field of fourteen inches in diameter 

 (measured at the usual distance — ten inches), equally distinct 

 and well illuminated over every part, and admirably adapted 

 for the display of sections of wood, wings of insects, and 

 objects of a similar description, and also for opaque objects. 

 When employing it for these purposes, he prefers using a 

 single lens as an object-glass instead of an achromatic one, as 

 the latter are adjusted for a much smaller field of view, and 

 produce an image which is distinct only in the centre." 



The power of the microscope may be greatly increased, not 

 only by using different eye-pieces and object-glasses, but also 

 by increasing the distance between them by the draw-tube, 

 described in page 69. For instance, suppose that the one 

 inch object-glass and the lowest eye-piece are employed, and 

 that without the draw-tube the magnifying power was thirty- 

 five, and with the second eye-piece forty-five, and with the 

 third eighty-five, by the draw-tube these powers may not 

 only be made even numbers, as forty, fifty, and ninety, but all 

 intermediate numbers, if required, may be obtained without 

 changing either the eye-piece or the object-glass. Besides 

 this, it is often required to make a particular object fill the 

 whole or nearly the whole of the field of view ; this is readily 

 done by employing the eye-piece and object-glass that most 

 nearly eifect it, and then accomplishing the remainder by 

 drawing out the tube. In the use of the micrometer eye- 

 piece, the draw-tube wUl also be found of essential service to 

 make the divisions come out whole numbers, a point that will 

 again be more particularly dwelt on. 



* Art. " Mioroscdpe," Cyclopcedia of Anatomy and Physiology, p. 342. 

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