148 



KNOWLEDGE. 



April, 1913. 



slips can be got at directly, but if either of those at the 

 ends (right and left) is wanted we have to remove one of those 

 in the centre. 



Figure 145 shows one of the empty trays, made thus. 

 First cut the base piece to fit easily inside the box. Now lay 

 four plain three-inch by one-inch 

 glass slips, centrally, side by side, 

 on the base piece, and run a pencil 

 line round the lot. Then cut four 

 strips of card (of thickness a trifle 

 more than the thickness of the 

 glasses) of such size as to fit the 

 card just outside the pencil line. 

 These strips will probably be 

 between one-eighth and three- 

 sixteenths of an inch wide, but 

 their actual size will depend on 

 the size of box in use. (Boxes for 

 quarter-plates vary a trifle in inside 

 measurement.) These four edging 

 strips are fixed to the base piece. 

 Then across each of the four 

 corners is fixed on a triangular 

 piece, so that the bottom or 



underside of the base piece may be safely clear of the cover 

 glasses on the slips. 



Practical points: (1) Use a sharp knife, preferably one with 

 fixed blade like an office knife. (2) Use a flat metal straight edge 

 for guiding the knife. (3) For cutting on, a piece of card is 

 good — perhaps the best of all things, as it does not blunt the 

 cutting knife point like metal or glass, and the card which is 

 being cut does not slip about. (4) For a fixing agent I vastly 

 prefer fish glue or seccotine. In the bottom of a small wine 

 glass put, say, half a salt spoonful of seccotine, add about one 

 third as much water and work up the mixture with a cheap 

 (penny) paint brush. This is also a 

 good tool for applying the adhesive. 

 (5) When one piece of card has been 

 stuck on to another lay the two 

 together in an old book for a few 

 minutes, to keep all flat until all is 

 fairly dry. (6) Do not use too much 

 adhesive. If any is squeezed out 

 between two pieces of card it is a 

 sign that too much is used. This 

 does not give such a good joint as 

 only just enough to cover the two 

 touching surfaces. 



F. C. Lambert, M.A., F.K.P.S. 



illuminators can be secured without expenditure other than 

 that of a small amount of time. -. 



QUEKETTER. 



DARK GROUND ILLUMINA 

 TION. — It may be of interest to 

 your readers to know that a dark 

 ground illuminator, for use with high 

 power objectives, can be made without 

 the underside of the top lens, as shown 



grinding away 

 in the piece 

 of apparatus computed by Mr. Nelson in your March 

 issue. All that is necessary, is to place between the 

 top and next lens of the Abbe Illuminator, an opaque disc of 

 such a diameter as will cut off all the rays that directly enter 

 the objective. The simplest way is to take some tin foil and 

 begin by making a disc within a fraction of the diameter of the 

 upper side of the second lens — that is, the lens immediately 

 behind the front lens — of the condenser. This disc is then 

 rested on the upper side of the second lens and made to stay 

 in position by means of a little immersion oil or similar 

 material. The top lens is then screwed on and must be in 

 immersion contact with the underside of the object slide. 

 Now, if a one-sixth inch or one-eighth inch objective be used 

 on the object, which must be mounted in a medium other than 

 air, it will soon be seen whether the object or particles are lit 

 up with a black background. If no light passes, reduce the 

 size of the tin foil disc very slightly and repeat the experiment 

 until the desired effect is obtained. Each individual disc can 

 then be kept for the different objectives and a result equal to 

 that obtainable with the expensive immersion dark-ground 



ON THE RELATIONSHIP OF APERTURE TO 

 POWER IN MICROSCOPE OBJECTIVES. — One 



would have thought that the 

 late Dr. Dallinger had settled 

 the question of the value of 

 aperture once and for all 

 by means of the photomicro- 

 graphs, Nos. 7 and 8, on the 

 frontispiece to the last edition of 

 Carpenter ; but it seems an 

 obsession with some minds to 

 deny the obvious on this subject. 

 We can pass over Mr. Hutton's 

 references to dilettanti and ignora- 

 muses as opposed to workers and 

 savants and to opticians playing 

 to the gallery, as having no place 

 in a scientific discussion ; but he 

 should at least have stated the 

 Figure 144. facts correctly before indulging in 



such expressions. 

 He gives the limit of keenness of vision as one hundred and 

 twenty-five lines to the inch because few eyes can measure closer 

 than this unaided. The question is not what the eye can measure 

 or count, but what it can perceive, and most eyes can easily 

 separate lines as close as a tenth of a millimetre. If anyone 

 doubts this, let him observe the scale on an eyepiece micro- 

 meter, with this ruling, in ordinary daylight. 



He also assumes that no objective will bear more than a ten 

 eyepiece. However true this may be for the highest powers, 

 it increases up to at least twenty, or even twenty-seven, with 

 the lower powers of the same series. 



His fundamental error, however, 

 is with regard to the total magnifi- 

 cation. The initial power of an 

 objective is always taken for an 

 image distance of two hundred and 

 fifty millimetres, and the image is 

 formed at approximately this distance 

 with the English tube, so that the 

 eyepieces being marked with their 

 actual amplifying power, the total 

 magnification is obtained by simple 

 multiplication. With the continental 

 tube the actual size of the image is 

 only about two-thirds of this diameter, 

 and, to make the result uniform, 

 the eyepieces are marked with 

 only about two-thirds their actual 

 amplifying power — a ten eyepiece 

 being really fifteen — so that simple 

 multiplication still gives the correct total. 



Mr. Hutton has given the actual objective magnifications 

 for the short tube; but has taken the nominal figures of 

 the eyepieces as real and has, therefore, worked out his table 

 of apertures at two-thirds of the required figures. 



A glance at Professor Abbe's table quoted by him on page 64 

 of Knowledge for February will show that the magnifications 

 are given for an image distance of two hundred and fifty 

 millimetres and the list of eyepiece powers given on page 63 

 are certainly only nominal. 



This extra eyepiece amplification with the short tube makes 

 no difference to the quality of the final picture, as the 

 objective image is obviously correspondingly concentrated ; 

 but the objective must, of course, be corrected for the tube 

 length with which it is used. 



Stated properly, the table becomes as follows with an 

 uniform eyepiece amplification of ten. I have given the 

 necessary apertures for a keenness of perception of both 

 one hundred and twenty-five and two hundred and fifty lines 

 to the inch. 



It will be seen that even with an eye of half the usual 

 keenness and a ten eyepiece, the necessary N.A. for a one- 





Figure 145. 



