24 Transactions of the Society. 



the greatest apertures, is at least 12-15, and considerably higher with 

 the medium and low objectives. A total magnifying power of 1200 

 requires therefore no more than an objective amplification of 80-100, 

 and consequently it can now, with the help of higher eye-pieces,* be 

 obtained with an objective of 3 to 2 "5 mm. focal length, whereas it 

 was formerly only possible with a focal length of 1 • 25 mm. 



Even if but little value is attached to the removal of the incon- 

 venience which unavoidably accompanies the use of objectives of very 

 short focal length, there is one essential advantage gained at any rate : 

 the limits within which each objective may be used are very materially 

 extended, for a series of very different amplifications may be obtained by 

 merely changing the eye-piece. It is clear that an objective of 3 mm. 

 focal length, which by means of an eye-piece of suitable strength secures 

 a magnifying power with good definition of 1200-1500, is in this respect 

 of greater value than an objective of the much shorter focal length 

 which has hitherto been necessary, because the former includes in 

 itself the performance of a medium objective when a low eye-piece is 

 employed. 



Thirdly and lastly, the realization of an achromatism of higher order 

 in microscopic objectives is of particular value in relation to photomicro- 

 graphy, because the correction errors of the ordinary achromatic systems 

 exercise a disturbing influence in this case to a much greater extent than 

 in observation with an eye-piece. As a result of these there is not only 

 a considerable difference of focus between the optically and chemically 

 active rays, which renders the correct adjustment for the photographic 

 focus very doubtful, but in addition, since the spherical correction of the 

 objective can only be effected with certainty for the brighter visible light, 

 there always remains a marked spherical over-correction for the chemical 

 rays which lie near the violet end of the spectrum, and on account of 

 this the concentration of rays cannot be made so complete in the photo- 

 graphic image as with eye-piece observation. Both these defects are 

 remedied in the apochromatic objectives, the former by the removal of 

 the secondary colour-deviation, the latter by the production of a uniform 

 spherical correction for all colours. Those objectives will therefore 

 insure that the best chemical image shall lie in the same plane with the 

 best oi^tical image, and that the action of the former shaU be as perfect 



* "With rej^ard to the very general idea that the use of strong eye-pieces is in itself 

 disadvautageous — that they involve loss of light, and that it is therefore essentially 

 necessary for higli magnifying powers to employ objectives of short focal length and 

 low eye-pieces — it may be remarked that this view can neither be optically justified, nor 

 does it correspond to a rightly interpreted experience, but has arisen in an unwarranted 

 generalization from certain observations. " Dark" images are given by high eye-pieces 

 if their use gives a too great (empty) magnifying power, i. e. if the total magnifying 

 power rises above that value for which the details of the image, as determined by the 

 aperture of the objective, are exhausted for the eye ; and also if the concentration 

 of the rays by the objective is so incomplete that it docs not admit of the full magnifying 

 power without at the same time making the defects visible. If neither of tlicse con- 

 ditions liolds, the subjective impression of brightness is not alftcted, whether the 

 magnifying power is obtained by the use of a strong objective with a low eye-piece, or 

 by a weaker objective o/ the same aperture and a high eye-piece. The physical bright- 

 ness of the image in every case depends only upon the aperture and the total magnifying 

 power, and it is of no account in what way the latter is produced by means of focal 

 length of the objective, length of tube, and focal length of eye-piece. 



