102 A New For nnila for a Microscope Object-glass. 



by any alteration in the radius of the lenses. By projecting the 

 blue and red, or visible rays of greatest and least refrangibility 

 through the system, the cause became apparent. The left-hand 

 section of this object-glass is shown in Fig. 6. The rays from the 

 focus are slightly divided by the first front surface. On emerging 

 from the back the separation is increased ; the red ray (r) is out- 

 wards, and the more refrangible or blue ray (b) inwards. Next, 

 the divergence of these two rays is extended by the middle single 

 lens. The following crown lens extends the angle of divergence so 

 far that the flint lens of the back triple cannot recombine them ; 

 and they emerge at two distinct zones, shown by the practical test 

 of the " artificial star " or light-spot reflected from a mercury 

 globule, viewed within and without the focus. 



It might be supposed that these rays at their final emergence 

 can be so refracted as to project the blue outwards. A crossing 

 point would then occur at a fixed conjugate focus in the body of 

 the microscope, at which all rays would be combined ; and if this 

 focus was adjusted to that of the eye-piece, achromatism and final 

 correction would be the result. But to meet the various conditions 

 occurring in the use of the microscope, the conjugate focus con- 

 stantly alters in position, this being affected by every change of 

 eye-piece, length of tube, or adjustment for thickness of cover ; 

 therefore a correction for a fixed point cannot be maintained. 

 Achromatism in the microscope object-glass, like that of other 

 perfectly corrected optical combinations, must be the reunion of the 

 rays of the spectrum close to the final emergent surface of the 

 system. The remedy suggested by these experiments appeared to 

 be a transposition, that is, in placing the over-corrected triple in 

 the middle of the entire object-glass ; this would at once cause a 

 convergence of the blue and red rays. A single lens of longer 

 focus at the back would then bring these rays parallel at the point 

 of final emergence. 



By projection in a diagram this condition was apparently 

 realized. The dispersive power of the flint (density 3 " 686) was 

 taken by the refractive index 1 ■ 76 of line H in the blue ray of the 

 spectrum, and 1*70 of line B in the red ray. The refraction of 

 the corresponding rays in the crown (density 2 ■ 44) was 1 ' 53 H 

 and 1 • 51 B. With these indices the rays are traced in Fig. 6. The 

 radii in the right-hand half-section are those of an " eighth " of the 

 new form drawn twenty times the size of the original. The single 

 front is of the usual form, as this is much alike in all cases. The 

 radius or focus of the single plano-convex back is about four and a 

 half times that of the front, and the focus of the middle (triple) 

 three times. The passage of the blue and red rays at the extreme 

 of the pencil is shown in contrast with the preceding, the separa- 

 tion from the same front being alike. 



