648 



NATURE 



[Oct. 31, 1889 



been made with both forms of instruments, which tend to 

 show that, as against the refractor of the ordinary con- 

 struction, the reflector can well hold its own, but that 

 while it is obviously impossible for the optician to improve 

 the field of the reflector, it is by no means impossible to 

 do so with the refractor, and time and patient experiment- 

 ing have shown that, by a modification of the curves of 

 an objective, equally good definition of the central pencil 

 can be obtained, combined with a very much better and 

 flatter field, so that, however well reflectors could compete 

 with the ordinary form of refractor, they cannot do so 

 with forms constructed with special reference to field. 



It should be borne in mind that the question of field is 

 one which the optician was never before asked to consider 



Fig. 7. 



in telescope objecti\ es. The field of such a sized telescope 

 used for visual woik would not be more than 4°, even 

 with the lowest power. It has been found possible to 

 obtain good definition over a field of 2° with either re- 

 flector or the ordinary form of refractor, and with the 

 modified form considerably more. This question of field 

 is, as I said, a very important one, for on it depends the 

 amount of time required to complete the survey. If one 

 instrument gives equally good definitions over 3° square, 

 i.e. nine square degrees, as another does over 2° square, 

 i.e. four square degrees, it is evident that the first instru- 

 ment, equally energetically worked, is capable of com- 

 pleting the survey in less than half the number of years 

 it is possible to do with the second instrument. 



There is one point connected with this question of field 

 which is of great importance. 



Various forms of objectives give various characters of 

 images of star disks at the edge of the field. Some give 

 a bright nucleus with a tail like a comet, some assume a 

 form approaching to a cross, and some give elliptical 

 disks. 



Of course perfection would mean absolutely circular 

 disks all over the plate, but when this cannot be obtained, 

 the last or elliptical disks are very much preferable to 

 either of the others. It is quite possible to fairly estimate 

 the most central point in an ellipse if the illumination 

 over it be tolerably equal, but in the case of the comet or 

 irregular form this is not possible. 



The newer forms of objectives are peculiar in that 

 the distortion of the lateral star images are of their least 

 objectionable character. 



It has been suggested that good results might be ob- 

 tained by using the new "Jena" glass with rational 

 spectra, but I have made inquiries respecting this, and it 

 is not considered by the makers themselves that this 

 glass in its present state would be suitable for the purpose 

 of these photographic objectives. Until the permanency 

 of that glass be thoroughly tested by exposure to various 

 climates for some years, it does not appear safe to use it 

 for such important work as this. There can be little 

 doubt, however, that for ordinary visual work this glass, 

 if capable of being made into large disks, will allow of the 

 production of objectives superior to anything hitherto 

 made. 



I ought perhaps to mention that it is possible to make 

 an objective which can be adjusted to work either as a 

 photographic or visual objective. 



By separating the lenses of an ordinary objective, the 

 chromatic correction can be so much reduced as to render 

 it suitable for photographic work, but unfortunately the 

 spherical aberration is reduced at the same time and 

 definition destroyed. 



Prof. Stokes, however, suggested to me that by con- 

 structing an objective in such a manner that the crown 

 lens would be unequally convex by a certain amount, 

 and that the spherical aberration would be correct when 

 the flatter side was outermost, that then the chromatic 

 aberration could be reduced as before by separating the 

 lenses, and the reduced correction for spherical aberration 

 raised by turning the crown lens with its more convex 

 side outerwards. 



This I tried, and found act perfectly. 



I described this last spring at the Royal Astronomical 

 Society, but it has since been re-invented in America. 



It remains for me now only to describe how these new 

 photographic objectives are tested. 



The testing of objectives for visual work is a matter 

 almost altogether of eye experience. 



In these photographic objectives a diflerent course 

 must be adopted, as the image appears to be badly 

 corrected to the eye when it is rightly corrected for 

 photographic rays. 



The Paris Congress decided that the rays G of the 

 spectrum should have in their objectives a minimum 

 focus; how is the correction to be verified.'* It is usual 



