ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 825 



metre divltlecl into half-millimetres ; tlie four central divisions are sub- 

 divided into lOtbs of a millimetre ; it is engraved on horn. For strong 

 magnifications the micrometer engraved on glass has a length of 

 1 millimetre divided into fiOths or into lOOths of a millimetre. 



In order to see, before making a determination, that the camera itself 

 gives rise to no magnification, at A' B' is placed a second scale 1 metre 

 in length, of which the middle point M' is at the same distance from the 

 camera as the middle point of the other scale. When M' F' is exactly- 

 equal and perpendicular to N 0, the eye placed at N sees whether the 

 virtual image of the scale seen by reflection in the camera is exactly 

 superposi-d over the first scale seen directly. 



A camera lucida in which only one reflection occurs, is more suitable 

 than one in which two reflections take place, such as that of Oberhausser 

 and Xachet, since in the former the exact point N occupied by the 

 summit of the cone of reflected rays is known, whereas in the latter a 

 graphical construction would be necessary to determine it. The author 

 di-aws attention to the error which can follow from neglect of this point 

 and demonstrates how with cameras like those of Xachet and Oberhausser 

 t«-o equal figures drawn in the same plane can never be exactly superposed 

 for an eye which receives them both at the outlet of the camera. 



In making a determination, the dioptric apparatus to be examined is 

 placed in front of the camera with its optic axis F' M' at right angles to 

 the line of vision N 0, and passing through the middle point M of the 

 micrometer. A preliminary observation by the aid of the sun's rays gives 

 the position F' of the second principal focus. The distance of the 

 diopter from the camera is then regulated so that the focus F shall, 

 after total reflection and deviation through 90°, coincide with the nodal 

 point X of the eye. For systems of very short focus, the eye is armed 

 with a spectacle-glass, as recommended by M. Monoyer. 



The number of divisions are then read off on the scale which are 

 exactly covered by a given number of divisions of the micrometer. The 

 ratio of these two gives the magnification at 5 metres. In taking the 

 fifth we have in dioptrics the power of the system. 



The preceding method is not applicable to the eye-piece on account of 

 the image being virtual. The difliculty is obviated in the following 

 way. Observation is taken of the magnification of the Microscope with 

 the eye-piece in place, which gives, say, dioptric power $i. A second 

 observation is then taken with the eye-piece completely drawn out, 

 which gives a second dioptric power 4'2. The extent of the di'awing out 

 of the eye-piece is measured. The formula for a system of two lenses of 

 dioptric power Fj and Fg, and at distances d^ and cl^ apart, gives 



$, = 8,F, F, = d, F, F, - (F, + F^) 



whence by subtraction 



$,_$, = {B, -81) F. F2 = (d, - di) F1F2 



and 



F, = 



(82 - 80 F, - id, - d,) F, ' 

 For any system of lenses on the same axis can be substituted 



