174 W. K. BRIDGMAN ON THE PRINCIPLES OF 



to direct the rays at the proper angle of incidence upon the object, 

 no better means of illumination can be desired either for effect or 

 for convenience. 



In applying it, the lamp may be placed, as usual, on the left-hand 

 side of the instrument, and the mirror should be turned a little to 

 the right and the rays directed on to the outer edge of the Lieber- 

 kiihn on the same side as the mirror ; after which a very slight 

 movement of the mirror will suffice to bring the best part of the 

 flame on to the object. When rightly adjusted the light will be full 

 and clear, and perfectly free from haze or glare. An excellent test 

 for obtaining this is afforded by the underside of the leaf of the 

 Blumenbachii coronata, the delicate transparent hairs of which are 

 almost invisible under an imperfect illumination, but, under a proper 

 light, stand out distinct and fresh, like herbage after a shower of 

 rain. 



To prepare a scale for obtaining this angle, in the absence of a 

 sector, the following plan will be found sufficiently accurate : — 

 Procure a large sheet of paper and fold it in two, so as to obtain 

 the base line a, A, b (Fig. 1) at one edge, and, perpendicular to it, 

 the median line A B. Then, with a pair of compasses opened to 

 8.y inches, draw the semi-circle c, d, e. Closing them to 3^- inches, 

 mark that distance on each side of d, and draw the lines C A, A I), 

 which will give C A B, B A D at the angle of 56° on each side of 

 the perpendicular A B. Repeating the same operation with the 

 compasses opened to 3| inches, will give E A B, B A F angles of 

 54 o5\ the polarizing angle of glass. Let any small card or paper 

 now be placed, with a marked point to coincide with A upon this 

 diagram, and let the lines be continued on it to the point A, and it 

 will then serve as a guide for obtaining the angles with the instru- 

 ment. On placing the card upon the stage behind the object glass, 

 and with the median line behind the axis, the mirror or condensing 

 lens may then be easily placed in the proper position, as at Fig. 2, 

 and the light afterwards brought to its corresponding place, as there 

 shewn. 



In viewing objects by transmitted light the same law equally 

 applies ; every cone of rays necessarily contains, at its axis, certain 

 portions that are either vertical, or so nearly so, as to suffer very 

 little refraction, and hence, are too direct to be other than detri- 

 mental to distinct vision. In practice, therefore, the central portion 

 should be " stopped out " by a suitably shaped diaphragm, and the 



