TEST OBJECTS. 475 



6, are all exceedingly useful in their way, to show how by 

 very oblique pencils of light, with glasses of small aperture, 

 dots closely approximated may be converted into lines. 



Another very good test of the defining power of a micro- 

 scope is the ultimate structure of voluntary muscular fibre, 

 about which many differences of opinion have been raised. 

 The most excellent specimens of this beautiful structure that 

 have yet been shown, are those prepared by Mr. Lealand, 

 from one of which, with his kind assistance, figs. 10, 11, 12, 

 have been been drawn by Mr. Leonard. Fig. 10 represents 

 a portion of a muscular fibre or fasciculus of a pig, magnified 

 600 diameters, which has been so far separated as to exhibit 

 the structure of the ultimate fibres or fibriUte. Fig. 11 is a 

 specimen taken from another part of the same preparation, 

 but magnified 1,200 diameters ; in this it will be seen that 

 each fibril is composed of alternate bands or stripes of two 

 distinct structures ; but on more careful examination, a trans- 

 verse line will be found between each dark band, which gives 

 to the fibril an appearance of being composed of a linear series 

 of more or less oblong or square cells, with a dark substance 

 in the centre of each, as shown in fig. 12, in some cases as in 

 fig. 1 1 ; the transparent cell wall cannot be easily seen, the 

 dark substance extending as far as the sides of the cell. 



Noherts Tests. — M. Nobert, of Griefswald, having occupied 

 himself for some years in the manufacture and the testing of 

 a large compound microscope, discovered that the productions 

 of nature, which had been almost exclusively used as test 

 objects, were more or less different in the nature and arrange- 

 ment of their markings, hence he was led to the employment 

 of such objects for comparison as can be reduced to number 

 and measurement, as modern philosophy requires in all its 

 parts. The plan adopted by M. Nobert, is to etch on glass 

 ten separate bands at equal distances : each band is composed 

 of parallel lines of some known fraction of the old Paris line ; 

 in the first band they are -j^Vo' ^^^ ™ ^^^ ^'^ i^^wu of ^^^ 

 same quantity, whilst the intermediate groups, with regard 

 to the distance of their parallel lines, form parts of a geo- 

 metric series; these have been kindly furnished by Mr. De 



