126 ACCESSORY APPARATTP. 



vention of the celebrated anatomist Soemmering, is preferred by 

 some microscopic delineators to the cataera lucida. The fact is, 

 however (as the author can testify from his own experience), that 

 there is a sort of "knack" in the use of each instrument, which 

 is commonly acquired by practice alone ; and that a person habi- 

 tuated to the use of either of them, does not at first work well 

 with another. A difierent plan is preferred by some micro- 

 scopists, which consists in the substitution of a plate of neutral- 

 tint or darkened glass for the oblique mirror ; the eye receiving 

 at the same time the rays of the microscopic image, which are 

 obliquely reflected to it from the surface of the glass, and those 

 of the paper, tracing-point, &c., which come to_ it through the 

 glass. It is so extremely usefal to the microscopist, to be able to 

 take outUnes with one or other of these instruments, that every 

 one would do well to practise the art. Although some persons 

 at once acquire the power of seeing the image and the tracing- 

 point with equal distinctness, the case is more frequently other- 

 wise ; and hence no one should allow himself to be baffled by the 

 failure of his first attempt. It will sometimes happen, especially 

 when the prism is employed, that the want of power to see the 

 pencil is due to the faulty position of the eye, too large a part of 

 it being over the prism itself. "When once a good position has 

 been obtained, the eye should be held there as steadily as possi- 

 ble, until the tracing shall have been completed. It is essential 

 to keep in view, that the proportion between the size of the 

 tracing and that of the object, is afi:ectecl by the height of the eye 

 above the paper; and hence that if the microscope be placed 

 upon a support of different thickness, or the eye-piece be elevated 

 or depressed by a slight inclination given to the bodj", the scale 

 will be altered. This it is of course peculiarly important to bear 

 in mind, when a series of tracings is being made of any set of 

 objects which it is intended to delineate on a uniform scale; or 

 when the camera lucida (or any similar arrangement) is employed 

 for the purpose of Micrometry. All that is requisite to turn it to 

 this account, is an accurately divided stage-micrometer, which, 

 being placed in the position of the object, enables the observer 

 to see its lines projected upon the surface upon which he has 

 drawn his outline ; for if the divisions be marked upon the paper, 

 the average of several be taken, and the paper be then divided 

 by parallel lines at the distance thus ascertained (the spaces being 

 subdivided by intermediate lines, if desirable), a very accurate 

 scale is furnished, by which the dimensions of any objects drawn 

 in outline under the same power may be minutely determined. 

 Thus if the divisions of a stage-micrometer, the real value of each 

 of which is l-200th of an inch, should be projected with such a 

 magnifying power, as to be at the distance of an inch from one 

 another on the paper, it is obvious that an ordinary inch-scale 

 applied to the measurement of an outline, would give its dimen- 

 sions in two-hundredths of an inch, whilst each fifth of that scale 



