sixth inch is well towards what the best makers give, and for 

 a one-eighth inch and one-twelfth inch it is beyond the limit of 

 construction for dry and immersion lenses respectively, whilst 

 for a keen eye we already want an immersion for the one- 

 fourth inch, and with a one-twelfth inch could perceive more 

 than double the structure which it is possible for any cedar oil 

 immersion lens to resolve. For the lower powers a fifteen 

 eyepiece would enable us to use more aperture than any maker 

 has ever offered or could construct in a mount with the society 

 thread. Mr. Hutton's argument that high aperture is futile 

 because the eye cannot use it, therefore, fails completely. 



His ideas as to the limit of useful magnification are equally 

 faulty. His formulae with the various denominators -26, -13 

 and -10 simply mean that eyes with the respective keennesses 

 of 250, 125 and 96 lines to the inch will require magnifications 

 of 577, 1154 and 1500 diameters respectively to define 

 structure as minute as N.A. 1-50 can resolve. There is no 

 need, however, to strain the eyes always up to the limit of 

 perception, and a sensible man will gain relief by using a 

 higher eyepiece whenever the objective will bear it. Thus, a 

 keen eye would require a five eyepiece to see what a one- 

 twelfth inch of N.A. 1-50 can resolve (120 X 5 = 600) ; but 

 would certainly use an 8, 10 or 12 for comfort. The point as 

 to empty magnification is that the limit aperture of N.A. 1-50 

 can as easily be given to a one-twelfth inch as to a one- 

 fiftieth inch, and as the eyepiece amplification required for 

 a one-twelfth-inch is only moderate, objectives of higher 

 power are quite unnecessary, besides being more difficult 

 to make and use, and far more costly. 



The showing of the beads on Amphipleura pellucida has 

 nothing to do with the magnification further than is sufficient 

 to make them large enough to see. The lines usually seen are 

 from 92,000 to 95,000 to the inch, and require only N.A. -96 

 to • 99 to resolve them, although they are usually shown with an 

 oil immersion of N.A. 1-3 and oblique light as an exaggerated 

 diffraction effect. I have frequently seen them on what must 

 be a very coarse specimen, mounted in realgar, with the Zeiss 

 apochromats six millimetres for the long, and four millimetres 

 and three millimetres for the short tube, all of N.A. -95, with a 

 fourteen (actual) eyepiece for the first two and a ten for the 

 last. The images are absolutely identical, thereby proving 

 that the resolution is due only to the aperture ; but this 

 specimen cannot exceed 90,000 lines to the inch. The resolu- 

 tion of the beads is a far different matter. The cross lines 

 forming them are, according to the late Dr. Van Heurck, 

 127,500 to 130,050 to the inch and require in theory N.A. 

 1 -33 to 1 -35 to resolve them, so that they cannot possibly be 

 seen with N.A. 1-32 and any magnification. Visibility too, is 

 quite a different thing from resolution with such minute trans- 

 parent structures, and I believe they have never been 

 demonstrated with less than 1-40 N.A. aided by green light. 

 Dr. Spitta's plate is quite as good an example of fine manipula- 

 tion as it is of photography. 



Mr. Hutton makes another mistake with regard to the 

 resolution given by photography in excess of white light. The 

 thirty per cent, is for the extreme violet and would require a 

 specially constructed objective. Blue light gives only eight 

 and a half per cent, over white, and the eye could perceive the 

 extra resolution as well as the photographic plate if it could 

 bear the intense illumination, or had the cumulative power. 



The statement, amongst others, that Messrs. Leitz's photo- 

 graph at 1150 would give as much as the naked eye at 1400 

 is due to a misconception. The fact that N.A. 1 • 32 should 

 give as much resolution with blue light as N.A. 1-43 with 



white, or rather yellow-green, is probably the statement 

 sought for. The magnification does not affect it. 



The fact that some medical students prefer low aperture 

 objectives for their greater working distance and depth of 

 focus is no excuse for decrying so vital a property as aperture 

 on false premises. Those students who take their microscope 

 work seriously and desire to see all the structure there is in 

 their specimens will certainly obtain the highest apertures they 

 can afford in their objectives as soon as they realise that 

 resolution absolutely depends upon it, and will quickly learn 

 to substitute depth of focus by the use of the fine adjustment 

 and the natural accommodation of their eyes. 



Nor need either cost or working distance deter them. It is 

 true they cannot as a rule afford apochromatics or even semi- 

 apochromatics, and it is not necessary. First class lenses of 

 high aperture can be bought at ridiculously low prices, and 

 are so near perfection in both performance and workmanship 

 that they are used for everyday work by eminent workers. 



A two-third inch of a N.A. -30 for fifteen shillings and a 

 one-sixth in of N.A. -82 or an one-eighth inch of N.A. -85 

 for thirty shillings each can surely not be called expensive. 

 The first and either of the second will fill the battery for dry 

 lenses as they will each bear high eyepieces, whilst the 

 working distance of the one-eighth inch is fully two-fifths of a 

 millimetre over a No. 3 cover (quarter millimetre) and that of 

 the one-sixth inch a little greater. The student who is 

 clumsy enough for this to be a bar has a lot to learn before 

 he becomes a useful member of his profession, and if forty- 

 five shillings is considered too much to spend on them I 

 should like to know where he can get anything fit to work 

 with for less money. These are by a famous continental firm 

 and he cannot even get the lower apertures better than play- 

 things anywhere for less ; but if he prefers English make, any 

 of the first class houses supply objectives of similar aperture 

 and equal performance for a few shillings more. 



It is the one-twelfth inch of N.A. 1-30 which is the 

 expensive item ; but there is no substitute, and in view of the 

 obvious fact that the corrections and workmanship must be 

 better in proportion to both power and aperture to give a 

 relatively as good performance and the smallness of the work 

 makes it difficult to get them even as good, there does not 

 seem much prospect of cheapening this item. 



G. E. Garrard. 



ORNITHOLOGY. 



By Wilfred Mark Webb, F.L.S. 



OVEN BIRDS IN THE ARGENTINE. — There is, 

 perhaps, no greater privilege for the naturalist than to 

 wander in foreign parts and observe at first hand the various 



Figure 146. 

 An Oven Bird's Nest in a typical situation in the country. 



