NOTES AND MEMORANDA. 53 



eacli other at very small distances, or from their breaking so easily, 

 and the difficulty of replacing them without the help of an expert. 



In a micrometer devised by M. G. Govi,* the threads are replaced 

 by the two edges of a slit made in a very thin layer of silver, gold, 

 platinum, or other metal deposited on the surface of a glass slide 

 which has its faces perfectly plane and parallel. Such metallic coat- 

 ings may be obtained sufficiently opaque with a thickness of less than 

 a hundred thousandth of a millimetre. The slit is made by a steel 

 tracer so lightly as not to touch the glass. The breadth of the slit 

 depends on the fineness of the tracer, its neatness not only on the 

 shape of the tracer, but also on the thinness of the metallic layer. 

 When broad slits are wanted it is best to remove the metal in parallel 

 furrows rather than attempt to attain the same result by a larger 

 tracer, which might produce slits with irregular edges. The metallic 

 layer should be placed on the side whence the image comes, so that 

 the rays which emanate from it, and the light which grazes the edges 

 of the slit, may have to traverse the same thickness of glass and 

 undergo the same modifications. 



In consequence of the extreme thinness of the metal coating, the 

 strongest eye-pieces give no sensible thickness to the edges of the slit. 

 There is therefore nothing to fear from the eflPects of parallax even 

 when the micrometer is applied to images placed in the extremities of 

 the field. 



There should always be a portion of the metallic coating removed 

 normal to the slit, so as to allow the observer to see the images 

 when they appear on the field of the Microscope, and when they pass 

 oif between the edges of the slit. It is a good plan to remove a little 

 less than half the opaque coating, leaving intact the other half where 

 the slit is. It is equally practicable to take off two equal bands of 

 metal at the two extremities of tiie slit, and leave only the central 

 zone in the field, which need not be very broad. 



If a number of slits of difterent width be made on one plate, it is 

 possible to avoid employing several micrometers. 



The extreme tenuity of the metallic coating, its opacity, its 

 rigidity, and the fact of its not altering under considerable thermo- 

 metric and hygrometric variations ; tlie possibility of making slits as 

 narrow or wide as desired ; the facility of substituting different slides 

 in the same frame, are advantages in this micrometer which should, 

 as it seems to me, induce observers to use it in place of the thread 

 micrometer. 



"Cell-Soul and Cellular Psychology-" — Professor Haeckel has 

 recently published his reply to the address on " The Liberty of 

 Science in the Modern State," delivered at last year's meeting' 

 of the German Association, by Professor Virchow. He states that 

 the views he expressed at Munich with regard to tlie soul of the 

 cell, i. e. " that we must ascribe an independent soul-life to each 

 organic cell," are but the natural consequence of Virchow's own 

 teachings, viz. of the very fertile application which Virchow made of 



* 'Comptes Rendus,' vol. Ixxxvii. p. 557. The micrometer as described is 

 intended more especially for meteorological observations. 



