REINICKE, "Z. NEUERN MIKROSKOPIE."'' 295 



this no apparent advantage is gained by an increase in their 

 number. 



With respect to the mode in which polarizers^ constructed 

 as above, can be adapted for use to the microscope, M. 

 Eeinicke enters into long details, in which however it is 

 hardly necessary to follow him, as the mode must be left to 

 be determined pretty much by the construction of the micro- 

 scope. All that appears requisite to remark is, that the square 

 tubes above described can very easily be inserted and fixed in 

 cylindrical ones of any size, made of thick paper, coiled up 

 and properly secured, and that these cylinders, which are 

 quite as serviceable as those made of brass, can be fitted for 

 use in any way that the ordinary ones containing Nichol's 

 prisms are. Owing however to the necessarily greater length 

 of the glass column, it is better to insert one of the polarizers 

 in the microscope tube between the ocular and objective, than 

 to place it in front of the former, by which the field would 

 of course be too much diminished for most purposes. 



2. The article on Nobert^s Test-Plate, and on Object- 

 Glasses of recent construction, consists chiefly of a translation 

 of the paper by Messrs. Sullivant and Wormley, which first 

 appeared in the 'American Journal of Science and Art^ for 

 January, 1861, and was given in the April number of this 

 Journal for the same year. M. Reinicke retains the opinion 

 expressed in the second part of his " Contributions,^^ that the 

 new objectives of Hartnack, so constructed as to require the 

 immersion of the lower lens in Avater, are the best he is ac- 

 quainted with, or are only equalled by a combination by Hasert. 

 These, he says, are the only glasses with which he has been 

 able to show the cross lines in a new test-object, Frustulia 

 Saxonica. As this is an object with which we are unac- 

 quainted, we are unable to express any opinion as to its value 

 as a test, but from what M. Reinicke states, it would appear 

 to be one of the most difficult of the class to which it belongs. 

 In order to obviate some misapprehensions regarding its 

 structure, which appear to have been entertained by many 

 to whom he had sent specimens, he communicates a few 

 remarks concerning it. The Frustulia is a diatom, in form 

 not unlike a Navicula. Like all other diatoms, it is composed 

 of two siliceous valves, joined together into a double plate by an 

 interposed delicate frame. In all the specimens of this object 

 furnished by him, the valves are more or less separated from 

 each other according to the method proposed by Gersten- 

 berger. The valves and frame are accordingly found in various 

 degrees of disjunction, whence are produced, besides broken 

 fragments, three kinds of figures. 1. (a) A simple outline 



