ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 135 



species to another, and what is not to be overlooked, that it varies con- 

 siderably during the survival and decay, and during tho process of 

 hardening. In one preparation of living muscle-fibre from Hydrwphilus 

 I saw fibres in which the discs Z and E were well developed, by the side 

 of others in which the distinction could not be seen. 



With this want of constancy it seems to me to be dangerous to regard 

 the fibre with nine layers as the type,* without granting that there also 

 exist fibres with two layers. I am rather inclined to see the type in the 

 fibres with two layers, and to regard the appearance of more layers as 

 something secondary." 



Method of Representing and Calculating the Magnification of 

 Microscopic Objects in the projected images-f — Dr. P. de Vescovi has 

 published a paper under this title, which seems to us to contain a great 

 many elementary facts and statements. Divested of these, the following 

 extracts appear to contain the pith of the paper. 



The statement of the amplification rarely corresponds to the truth, 

 and generally deviates widely from it, since the methods ordinarily 

 used to calculate and to indicate the enlargement are defective, or at 

 least fail in something. The amplifications given in the tables which 

 are supplied with Microscopes are mostly obtained by multiplying the 

 magnifying power of the eye-piece by that of the objective— an inexact 

 method. 



More exact are those who give the system of lenses used, and the 

 names of the makers of the Microscope ; but in this case if one considers 

 the factors (such as length of tube), which contribute to the variations 

 in size of the image, the indication is still inexact ; as it may easily 

 happen that with a given eye-piece and objective, and upon the same 

 instrument, different amplifications may be obtained either of the real or 

 of the projected image. 



" To remove all uncertainty and possible difficulties, it is necessary 

 that the explanation of every figure should give the following data : — 



(1) The eye-piece and objective used. 



(2) The maker of the Microscope. 



( 3) The length of the tube. 



(4) The true dimensions of the object. 



(5) The ratio of the dimensions of the object to those of its projected 

 image, or the amplification of the drawing. 



Example : — 



Eye-piece 3. Objective AA Zeiss. 

 Length of tube = 17 cm. 

 Greater diameter of the object = • 026 mm. 

 Amplification of the drawing = 95." 



Measurement of Magnifying-power of Objectives. 



[Replies to query by J. S. Hewitt, T. F. S., " Practical," E. M. Nelson, 

 E. Holmes, " Gamma Sigma," J. D. M., and " Decern."] 



Engl. Mech., XLVI. (1887) pp. 325, 341-2 (2 figs.), 3G5 (1 fig.), and 417. 



* So far as I know, no one lias done this. Different authors have rather founded 

 different types which always, however, have a considerable number of layors. 

 t Zool. Anzeig., x. (1887) pp. 197-200. 



