ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



1057 



The result is that the blue-iris limit is thrown right back, and a 

 shorter prism is obtained, having an equally wide field or wider. 



Fig. 227 shows the same method applied to a slightly longer piece 

 of spar, producing a " reversed " prism of precisely the same external 

 form as the ordinary nicol, and having indeed everything the same, 

 save the direction of the crystallographic axis, as a comparison of 

 figs. 225 and 227 will show. 



The method of " reversing " the section is of course equally 

 applicable to flat-ended nicols. If a piece of spar is first cut so that 

 the terminators are orthogonal to the long edges of the prism, it is 

 obviously just as easy to slice the prism with a section that is very 

 nearly perpendicular to the crystallographic axis as to slice it with 

 one that makes only 45° with it. 



This new method of construction may be regarded as a compromise, 

 for the sake of cheapness, between 



the method of Hartnack and the Fig. 228. 



older method of Nicol. 



Nachet' s Camera Lucida. — This 

 is now made by M. A. Nachet for 

 use with an inclining Microscope, 

 when it takes the form of fig. 228. 

 The difference between this and the 

 camera for a vertical Microscope is 

 principally that the surfaces of the 

 prism have been cut to the angles 

 necessary to produce an exact co- 

 alescence of the images when the 

 body-tube is inclined 45°. 



Instead of the small central , 

 prism of the older forms, M. Nachet 

 uses with all his cameras the thin 



coating of gold suggested by Prof. G. Govi, the reflecting power of 

 which is sufficient to give a clear image of the pencil, while its 

 translucidity allows the object to be seen at the same time. 



Apparatus for cultivating Plasmodia.* — In his observations on an 

 aquatic Myxomycete, Mr. H. Marshall Ward found that the plasmodia 



tended to the upper parts of the culture drop, and he therefore used 

 the apparatus shown in fig. 229, in which the drop d, containing 



* Stud. Biol. Laborat. Owens College, i. (18SG) pp. Gi-85 (2 pis.). 

 Scr. 2.— Vol. VI. 3 Z 



