228 Royal Society : — ■ 



practically this would be no disadvantage, as these surfaces are 

 united with Canada balsam, whose refraction is higher than the 

 crown ; so that the rays in this case must proceed with very little 

 deviation. But incidences beyond the angle of total reflection may 

 be considered detrimental, as they imply excessive depth of curva- 

 ture; this can be discovered by looking through the front of an 

 object-glass held close to the eye, any air-films in the balsam near 

 the edge of the lens appearing as opaque black spots. 



At the commencement of the present year the author caused 

 a* few object-glasses to be made, with a middle of the form of 

 fig. 5, the performance of which was very satisfactory. In this 

 the extreme rays pass at more favourable incidences, and within 

 the angle of total reflection. The upper lens is of dense flint. 



When the experiments on the single front were concluded, and 

 the remarkable corrective power of the triple back in conjunction 

 therewith had been proved, the next attempt was to make the 

 middle also a single lens, leaving the entire colour-correction to be 

 performed by the one biconcave flint in the back. After numerous 

 trials it was found that though something like over-correction or 

 negative aberration could be obtained with the back, in the degree 

 requisite for balancing the under-correction of the single middle 

 and front when set at the prescribed distance of the aplanatic focus, 

 yet by trial on the mercury globule all the results invariably dis- 

 played two separated colour-rings : these could not be combined 

 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 emer- 

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

 outwards, 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 eyepiece, 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 

 eyepiece, length of tube, or adjustment for thickness of cover; 

 therefore a correction for a fixed point cannot be maintained. 

 Achromatism in the microscopic 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 



