ZOOLOGY AND BOTAXY, MICROSCOPY, ETC. 339 



secondary spectre remaining uncorrected. With the new glass those 

 different colours unite at one point, a tertiary deviation being left only. 

 2. Formerly the spherical correction was confined to the rays of one 

 colour ; this correction being made for the middle part of the spectrum, 

 the systems remained under-corrected, spherically, for the red rays, and 

 over-corrected for the blue rays. Now the correction of sph. a'berr. is 

 obtained for two different rays of the spectrum at the same time, and 

 the objective shows the same degree of chromatical correction for the 

 central as for the marginal part of the aperture. (Of course, this higher 

 degree of correction is not given by the glass from itself — it requires 

 a very careful utilization of the optical properties of the various kinds 

 of glass at disposal, in order to fulfil all those conditions, and this was 

 not even possible except by means of a greater complication of the 

 constructions ; I was obliged to introduce five separate lenses (for the 

 aperture 1-4) instead of the four applied hitherto). 



" The objective at hand is constructed on the single-front-type. It 

 contains ten single lenses in five separate parts. Two only of these 

 ten lenses contain silicious acid ; the glasses of the other eight are 

 phosphates and borates— the Crown and Flint glass which has been used 

 by the opticians hitherto does not contain, as essential constituents, 

 more than six chemical elements, 0, Si, K, Na, Ca, Pb ; the lenses of 

 the l/8th contain, as essential components of the glass, not less than 

 fourteen elements." 



" I (lid not introduce a greater aperture than 1 • 40 in order to preserve 

 a convenient working distance— which, in fact, is =0*25 mm. = 1/100 in. 

 The two oculars sent with the objective are constructed with the aim 

 to compensate certain aberrations outside the axis, which cannot be 

 got rid of in the objectives (of wide aperture). The whole series of 

 objectives, high and low powers, shall be so arranged, that this com- 

 pensation is always obtained by the same series of oculars." 



This last letter, I think, will be accepted as setting out inter alia in 

 a remarkably lucid way the optical advantages obtained by the intro- 

 duction and employment of the Jena glass in optical constructions. 



Achromatoscope.* — S. R. Williams describes the arrangements 

 adapted to testing substances for achromatic combinations which have 

 been called achromatoscopes, because without the necessity of measuring 

 the refractive indices we can tell by inspection where there is achromacy 

 or not. The various systems consist essentially of devices for viewing 

 two spectra in juxtaposition in which one is a channelled spectrum from' 

 one film and the other is from another thin film. Any comparison 

 spectrometer may be used. The author's method has especial significance 

 when applied to thin films of various kinds of glass in order to select 

 the best achromatic lenses. Preliminary work in testing the achromacy 

 of various liquids would indicate that for some purposes liquid lenses 

 might be used to a great advantage. The liquids would have to be 

 held in glass containers which were ground to the proper curvature, but 

 whose walls were parallel surfaces. The author describes his apparatus 

 and the theory on which it is based. 



" Amer. Jouni. Sci., xli. (1916) pp. 101-11 (8 figs.). 



