THE ANALYSER. 313 



imaginary divergence of the cones of light need no contradiction 

 after what has been stated above. It is evident that the magnify- 

 ing power of the objective, as well as the size of the diaphragm 

 of the illuminating apparatus, under the relations given in the 

 Microscope, exercise practically no influence upon the inclination 

 of the emerging rays, but act upon the brightness only of the 

 field of view. The length of the body-tube, likewise, need not 

 enter into the consideration, as the differences it produces are 

 infinitesimal. It is also clear that the divergence of the optically 

 effective rays between the objective and the eye-piece must in all 

 cases be the same, for their path is in a straight line ; and that, 

 on the other hand, the section of the illuminated space gradually 

 increases upwards, and at length nearly equals the diameter of 

 the eye-piece. From this it follows, however, that it is quite 

 immaterial at what part of the body-tube the analyser is inserted, 

 on the hypothesis that the diaphragm is always of the same 

 relative size i.e., is in the same relation to the above-mentioned 

 section of the whole body of the rays. The application of the 

 Nicol at the lower end of the eye-piece, as recommended by 

 Harting, cannot possibly afford advantages which are not attained 

 if it be placed lower in the tube; moreover, this position is 

 attended with the practical inconvenience that the prism must 

 be considerably larger to produce an equivalent effect. 



We cannot recommend that the analyser be inserted between 

 the refracting surfaces, because the prior survey of the field of 

 view and the critical observation of particular objects are thus 

 rendered more difficult. It may be important in investigations 

 by polarised light, and it is always convenient to make a pre- 

 liminary examination of the preparation under bright illumina- 

 tion and with the greatest possible distinctness of the image 

 that is, without analyser, and after it has been brought into the 

 middle of the field of view to examine it with the analyser. We 

 know no better method for satisfying these requirements than that 

 already recommended by Talbot, 1 viz., to apply the upper Nicol 

 above the eye-piece. With regard to the perfection of the polar- 

 isation we have not, however, found that an analyser applied in 

 the body-tube transmits light with crossed position to a noticeable 

 degree ; the field of view was so completely darkened that we 



1 Poggendorfl's "Annalen," Bd. xxxv. (1835), p. 330. 



