824 SUMMAEY OF CUBEENT KESE ARCHES RELATING TO 



qi and q, denoting the successive distances of the object from the first 

 focal point of the Microscope, though only their difference, i. e. the 

 displacement of the object, need be known. 



In the experimental determination the apparatus employed comprises 

 a Helmholtz ophthalmometer A B (fig. Ill), and a diffraction bank A S 

 as constructed by Duboscq. On the bank directed parallel to the 

 horizontal optic axis of the ophthalmometer are three vertical supports, 

 carrying respectively the diojjtric system L L', a micrometer P, and a 

 screen of ground glass beyond which is the source of light S. To 

 apply the above formula a first magnification Gj made by L L' of the 

 micrometer Pj is measured with the ophthalmometer. The micrometer 

 is then placed at P^, and the second virtual image Pg' measured by the 

 ophthalmometer gives the second magnification Gg. The displacement 

 Pj P2 = ^2 — 2i ^^ read off on the bank. 



The rapid method for determining the dioptric power, which makes 

 use of the camera lucida to measure the magnification of a micrometer, 

 depends on tLe other expression for the magnification, viz. 



G, = 2/F = (//+/_d„)F. 



Two processes can be employed. 



(1) Keeping d^ constant and displacing the object, its image is 

 formed at a new distance IJ from the eye, and a second magnification is 

 given by 



G, = q.,F = (?,'+/_ eg F, 



wheDce by subtraction 



-p ^ G2 - Gi 



4' - h' 



In the experimental determination the image of the micrometer is 

 projected by means of the camera lucida on a plane containing a divided 

 scale, and the ratio of the lengths superposed gives the magnification. 



(2) The second process consists in making cIq = /, which reduces 

 the above formula to 



G = Z' F, 

 whence 



-?■ 



so that only one detemiination of the magnification is necessary. The 

 apparatus employed consists of a horizontal scale (fig. 112) one metre in 

 length, a WoUaston camera P, and a micrometer M, illuminated by a 

 source of light S situated on the other side of a screen of ground glass E. 

 'I'he scale, clearly graduated in centimetres and half-centimetres from 

 to 50 starting from the middle, is strongly illuminated by two gas- 

 burners placed at each end. The camera lucida is placed at the same 

 height as the scale on the perpendicular to its middle point, and at a 

 liistance from that point a little less than 5 metres, so that the eye at N 

 may be accurately at that distance from the scale. 



TLe reflecting face ra n of the camera is inclined at an angle of 45^ 

 to the axis N O, and one of the faces of the right angle only intercepts 

 half the cone of rays falling on the eye from the scale. 



'i he micrometer M has for weak magnification a length of one centi- 



