444 H. von Mohl on the Arrangement of the 



Fam. 4. NephthyacUe. 



Coral arborescent or expanded, fleshy, membranaceous, often 

 very cellular. Cell of the polypes covered externally with large 

 fusiform calcareous spicula. 



* Nephthya (Spoggodia). ? Alcyonidia. **Nidalia. ***C/arM- 

 laria. 



Fam. 5. Tubiporidse. 



Coral calcareous, tubular. Tubes united by transverse plates 

 formed by the expanded edges of the tubes bearing the buds. 

 Polypes cylindrical. 



Tubipora. 



XL VI. — On the Arrangement of the Polarizing Microscope in the 

 Examination of Organic Bodies. By Hugo von Mohl *. 



That polarized light is so rarely made use of in the microscopic 

 examination of organic bodies, principally arises from the cir- 

 cumstance that the German and French microscopes, which are 

 almost exclusively used on the Continent, are so badly arranged 

 as to be almost valueless for detecting double refraction in those 

 organic structures which act but feebly upon polarized light. 

 Hence, to mention a few instances, Ehrenberg was unable to 

 detect this property in the leaf-scales of Olea Europaa, and in 

 the silicious valves of the Diatomacese; nor could Schacht dis- 

 cover double refraction in the primary membrane of the cells of 

 plants ; and Pereira, who occupied himself so much with pola- 

 rized light, was unable to see the black cross in the starch-gra- 

 nules of rice ; whilst these structures under my polarizing micro- 

 scope form most beautiful objects, so that not only can I resolve 

 the lines in Pleurosigma angulatum into dots, as well as with 

 common light, but their six-sided form is quite distinct. 



Even in my first attempts to use this instrument in phyto- 

 tomic experiments, I found that a bright and distinct image 

 could be obtained only by exposing the object to very concen- 

 trated polarized light. The necessity of this is evident from the 

 fact that three-fourths of the light reflected by the mirror is 

 necessarily lost in its passage through the polarizing apparatus, 

 which loss must be considerably increased by the reflexion 

 taking place at the surfaces of the lenses of the microscope. 

 "When, moreover, it is remembered that the depolarizing action 

 of most organic structures is much more feeble than that of in- 

 organic crystals, it may be easily understood that the image of the 

 former is dull and imperfect, or that they remain invisible, if very 

 intense illumination is not applied. Experiment also convinced 

 * Translated from Poggendorff's Annalen, No. 9. 1859, p. 1/8. 



