ZEISS'S APOCHROMATICS 369 



in each of the other zones is also brought to the same focus, sav .'>() 

 in the outer zone, and 40 in the centre circle. The result is that 

 the whole achromatic lens gives a total of light, on the principle stated 

 above, of 30 + 70 + 40=140. In the apochromatic system, how- 

 ever (fig. 323, 2), we find in the intermediate zone three parts of the 

 .spectrum united ; that is to say, 40 + 30 + 15 = 85 ; and two in each 

 of the others, say, 40 + 30=70. Thus an apochromatic objective 

 will give 70 + 85 + 70=225. 



Recalling the suppositions we have made for the purpose of this 

 graphic presentation of a difficult subject, it will be seen that a non- 

 achromatic objective would give 40, an achromatic 140, and an 

 apochromatic 225 out of a possible iotal of 300. 



This illustration might be exceeded in severe accuracy, but 

 scarcely in simplicity, and it sufficiently explains from this point of 

 view alone the vast gain of the apochromatic system. 



It is interesting to note that, while the microscope in its earlier 

 form took its powerful position by borrowing achromatism from the 

 telescope, it has now led the way to the apochromatised state, which 

 without doubt it will be the work of the optician, in constructing 

 the telescope of the immediate future, to follow. 



We would beg the reader to bear in mind in the purchase of 

 objectives that, whilst the vitreous compounds with which Abbe's 

 beautiful objectives are constructed are now accessible to all opticians, 

 and whilst without these Abbe's objectives could never have been 

 constructed, yet it does not by any means follow that because an 

 objective is MADE with the Abbe-Schott glass it is therefore apo- 

 chromatic ; the secondary spectrum must be removed, and the spherico- 

 chromatic aberration balanced, or it is ' apochromatic ' only by mis- 

 nomer. It is another feature of these objectives, which it is import- 

 ant to note, that they are so constructed that the upper focal points 

 of all the objectives lie in one plane,. Now as the lower focal points 

 of the eye-pieces are also in one plane, it follows that, whatever eye- 

 piece or whatever objective is used, the optical tube-length will 

 remain the same. 



Professor Abbe has found 1 that in the wide-aperture objective 

 of high power there is an outstanding error which there is no 

 means of removing in the objective alone, but, as we have already 

 explained, this is left to be balanced by an over-corrected eye-piece. 

 As this peculiarity pertains only to the higher powers, a correspond- 

 ing error had to be intentionally introduced into the lower powers in 

 order that the same over-corrected eye-pieces might be available for 

 use with them. 



It appears worthy of note in this relation that one of the best 

 forms for the combination of three lenses is that kiio\vn as Steinheil's 

 formula, which consists of a bi-convex lens encased in two concavo- 

 convex lenses. It will be observed by reference to the figure illustrat- 

 ing the apochromatic lens construction (fig. 319) that this is largely 

 made use of. In some instances the encasing lenses possess sufficient 

 density, with regard to the central bi -convex lens, to altogether over- 

 power it, the result being a bi-convex triple with a negative focus. 



1 Chapter II. 



B B 



