THE MICROSCOPE. 187 



central, but one appearance is seen, providing the focus is exactly 

 adjusted and color corrections of the objectives be the same. 



It is well known that perfect color correction cannot be made 

 with the glass now used by opticians and that even the best objec- 

 tives show considerable residual color. The exact tint of color 

 depends upon the glass used in the construction of the objective 

 and the exactness of the correction for primary chromatic aberration. 



In most of our best objectives (being slightly under-corrected) 

 the residual colors are green and ruby red, and with proper spherical 

 correction the spines of the podura scale show of a ruby red with 

 green interspacing. Any change of residual color changes the ap- 

 parent color of the spines, but the form should remain constant with 

 proper spherical correction. 



Now, what is the shape of these spines ? The books throw but 

 little light upon the subject. However, the old idea of rows of dots, 

 triangles, etc., has given way to the spine theory. But what is the 

 shape of the spines ? In his work " How to See with the Micro- 

 scope " Prof. J. E. Smith says the small ends of them are to be 

 sharply defined; and in the illustration which he gives, the small 

 ends are represented as sharp points ! I have never yet seen an ob- 

 jective, which, when properly corrected, would show the small ends 

 as sharp points. Prof. Smith kindly loaned me his ToUes' J and ^^-, 

 both immersion lenses, and neither of them can be forced to show 

 the spines as represented in his work. 



Some persons seem to regard the spines as only attached at the 

 small end, as is diagramatically shown in fig. i. If this were so, it 

 is highly probable that they would become broken or bent from 

 their normal position by a slight rub; whereas we find scales which 

 have been scratched, and the spines still remain, but in an injured 

 condition. In all probability their true attachment is by nearly the 

 whole of their under surface as is diagramatically shown in fig. 2. 



In fig. 3 I have endeavored to show, by a camera lucida draw- 

 ing, the appearance under a strictly first-class objective in correct 

 adjustment. Here the spines are shown magnified seven thousand 

 six hundred diameters, by a Spencer immersion }i — a lens of rare 

 excellence and one which resolves No. 17 of Moller's balsamed plate 

 by central lamplight. 



The podura scale from which this figure was drawn is one from 

 the original Beck stock. I have been asked why the " Beck 



