954 THE MICROSCOPE AND ITS REVELATIONS. 



minute object, or of a small portion only of a larger one, the prisms are to 

 be removed by withdrawing the tube containing them ; the slides should 

 then be opened wide, and the object, or part of it, brought into the centre 

 of the field ; the vertical and horizontal slits can then be partly shut, so as 

 to inclose it, and if the prisms are then replaced, and a suitable objective 

 employed, the required spectrum will be seen unaffected by adjacent 

 objects. For ordinary observations, Objectives of from 2 inches to 2- 

 3ds inch focus will be found most suitable ; but for very minute quan- 

 tities of material a higher power must be employed. Even a single Ked 

 Blood-corpuscle maybe made to show the characteristic Absorption -bands 

 represented (after Prof v Stokes) in Fig. 65. 1 



90. Micrometric Apparatus. Although some have applied their 

 micrometric apparatus to the Stage of the Microscope, yet it is to the 

 Eye -piece that it may be most advantageously adapted. 2 The Cobweb 

 Micrometer, invented by Ramsden for Telescopes, is probably, when well 

 constructed, the most perfect instrument that the Microscopist can em- 



Upper half, Map of Solar Spectrum, showing Fraunhofer lines. Lower half, Absorption Spe> 

 trum, showing position of Bands in relation to lines. 



ploy. It is made by stretching across the field of an Eye-piece two very 

 delicate parallel wires or spider's threads, one of which can be separated 

 from the other by the action of a micrometer screw, the head of which 

 is divided at its edge into a convenient number of parts, which success- 

 ively pass-by an index, as the milled-head is turned. A portion of th& 

 field of view on one side is cut off at right angles to the filaments, by a 

 scale formed of a thin plate of brass having notches at its edge, whose 

 distance corresponds to that of the threads of the screw, every fifth notch 

 being made deeper than the rest for the sake of ready enumeration. 

 The object being brought into such a position that one of its edges seems 



1 For further information on " The Spectrum Method of Detecting Blood," see 

 an important paper by Mr. Sorby, in " Monthly Micros. Journal," Vol. vi. (1871), 

 p. 9. 



2 The Stage-Micrometer constructed by Fraunhofer is employed by many 

 Continental Microscopists ; but it is subject to this disadvantage that any error 

 in its performance is augmented by the whole magnifying power employed ; 

 whilst a like error in the Eye-piece Micrometer is increased by the magnifying 

 power of the eye-piece alone. Dr. Royston-Pigptt has pointed out (''Monthly 

 Micros. Journ.," Vol. ix., 1873, p. 2.), that by placing the Cobweb Micrometer at 

 some distance beneath the stage, and by forming an aerial image of it (by an 

 interposed lens) in the plane of the object ; the delicacy and accuracy of its 

 measurements may be greatly increased ; the numerical value of each division 

 being reduced, in proportion to the reduction in the size of the aerial image, 

 which will of course be determined by the focal length of the lens that forms it, 

 and by the distance of the Micrometer beneath it. 



