ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 



579 



Mr. E. T. Glazebrook also describes * a polarizing prism designed 

 to obviate tbe lateral displacement in the image produced by the Nicol 

 prisms and to give a field in which the plane polarization should be as 

 nearly as possible complete. It was not, however, designed for 

 microscopical work. 



Depth of Vision in Photomierography.t— Mr. G. E. Davis, in 

 an article on " Penetration in Objectives," calls attention to the differ- 

 ence that must necessarily exist between the appearance of a solid 

 object seen by the eye through the Microscope and the same object 

 in a photomicrograph. What is seen through the Microscope is the 

 result of the combined effects of the accommodation of the eye and the 

 focal depth of the objective, but when a picture is thrown upon a 

 sensitive plate it is evident that the first element is nearly eliminated, 

 and the only depth of vision attainable is that which the objective 

 itself possesses. 



The following table shows the focal depth of the objective, the 

 accommodation depth of the eye, and the total depth of vision for 

 objectives from 4 in. to l-20th in. (A eye-piece.) 



From this table it will be seen that large objects cannot possibly 

 be penetrated even with objectives of low angle and medium power. 

 The seeds of Betula alba measure 1100 /x, across, and require therefore 

 560 ^ of penetration to see the whole of one of them under one 

 focussing.^ This cannot be obtained from a IJ in. objective of 

 0-14 N.A., even allowing the 230 ^ which the accommodation of the 

 eye affords, and if we wish to photograph such an object, the 4 in. of 

 0-07 N.A. will not have sufficient focal depth. 



* Proc. Phys. Soc. Lond., v. (1883) pp. 204-16 (6 figs.), 



t Micr. News, iii. (1883) pp. 172-6. 



I i. e. half the depth — diameter in the case of a spherical object. 



2 p 2 



