No\-EMBER, 1903.] 



KNOWLEDGE. 



261 



illustrations, as well as a side view, showing the appearance of 

 the spring after it has been released. The sucker is supposed 

 to be used in resting, attaching the insect to an object ; it is so 

 rare in entomology, that those families possessing it, are grouped 

 in the sub-order Collembola. The claws are also provided with 

 a gland containing a viscid secretion, which runs down a minute 

 perforation, and enables the " S])riiig-tail " to adhere and walk 

 on glass head downwards. After I had kept the insects about 

 ten days several of them cast their skins. They are said to eat 

 them, but mine did not. The cell alluded to can be made thus : 

 Take a piece of thin wood or stout caidboard, two inches long 

 by one broad. Out of one of the ends, punch or cut a hole, 

 about half an inch from side to side. Cut several pieces of 

 blotting paper the same size as a glass slip (three inches by one 

 inch). Piuk blotting paper must be used, as white is bleached 

 by a chemical injurious to insects. Wet the blotting paper, place 

 it on the glass slip ; on this jilace the wood or cardboard in 

 such a manner that the hole or cell is in the centre of the slip. 

 After having placed the insects with some weed in the cell, cover 

 the opening with a square piece of cover-glass. Keep the whole 

 bound together with two elastic bands, or tie with thread. 

 This cell, if the blotting paper is I'aily wetted, will keep the 

 insects alive for a considerable time. The slip can be kept 

 under a bell-glass to prevent evaporation. — W. WfiScHt, f.r.m.s. 



Determining the Numeric.vl Aperture of Dry 

 Oli.iECTivES. — Several suggestions have been made for rendering 

 easy the exact determination of the N A. of objectives, but the 

 method attributed to Mr. Conrady, which appears in " Photo- 

 micrography," by E. J. Spitta, is ])robably one of the simplest. 



Briefly, the system recommended is as follows : — Two pieces 

 of white paper are laid upon a black background, their straight 

 inner edges set parallel to one another at a convenient distance 

 apart, say 20 cm. for lenses of '5 N.A. and over, but less for low 

 angled ones. 



A scale, or some accurate means of measuring, is held vertically 

 about mid-way between the two pieces of paper, and the 

 objective to be tested placed vertically against the edge of the 

 scale. It will now be noticed on examination of the back lens — 

 the eye being placed at a distance about equal to the tube length, 

 for which the objective is designed — that as the objective 

 approaches the table the interval between the images of the two 

 pieces of paper will widen until at last a point is reached where 

 only a slight bluish flicker remains visible on either side at the 

 extreme margin of the lens. 



This is an indication that the inner edges of the pieces of 

 paper are in the path of the most oblique rays that the objective 

 is capable of receiving, and the angle between these two points, 

 taken at the principul focus of the objective, is the angle of 

 aperture. 



This is now determined by reading off the vertical distance 

 from the table to the front of the objective in this position and 

 then subtracting the working distance of the lens so as to get 

 the distance from table to focus. 



This distance divided by half the distance between the two 

 pieces of paper is the co-tangent of the semi-angle of aperture. 

 The latter may be then obtained from a table of trigonometrical 

 ratios, and the sine of the same angle is the N.A. of the 

 objective. 



This method has proved wonderfully accurate and rapid, and, 

 without adding to the labour involved, an imjirovement may be 

 effected by using the microscope for holding the objective and 

 determining the various vanishing points by the rackwork. 



It will then be a very easy matter to measure the distance 

 between the sheets of white paper, which can be conveniently 

 laid on a piece of black cardboard ou the stage of the microscope 

 in front of the objective. 



It also facilitates the measurement of the working distance of 

 the lens. Of course no eyepiece would be used. 



This system is ajiplicable to dry lenses only. 



]\IiCi£i>S('Ol'E Tahi.e. — In the number of this journal for 

 November, 19()'2, some suggestions were made and devices 

 invited for a microscope table, suitable for work in a restricted 

 space, and in the January number a correspondent sent details 

 of one that ho was using. 



We have since receive<l further suggestions, but the best 

 emanates from Mr. B. II. Morgan, anil will be appreciated by 

 reference to the illustrations herewith, one showing the table 

 open and the other closed. 



The table top measures 2 ft. 8 in. by 1 ft. 3 in., and with the 

 top in position an extra shelf is available, 2 ft. 8 in. by 10 in. 



Microsco])e Table. 



This top rests on two brass tubes which push out from the sides. 



The eight drawers vary in size, the largest wiU take 10 oz. 

 bottlts, and the smallest 3 in. by 1 in. slips. 



The front is removed when in use, and when in position it is 

 held by two dowels, and one lock secures the whole. Such a 

 table could be mounted on a sewing machine stand, or on proper 

 bench legs. 



The advantages of such a table will at once be obvious, everv- 



Microscope Table. 



thing can be closed and left in position at a moment's notice, 

 and dust is excluded. 



PiiOTOcu.vriii.NG Im.\(;es in the Cornea of a Beetle's 

 Eve. — There are probably few objects which create so much 

 interest as the exhibition of multiple images in the cornea of a 

 beetle's eye, and correspondents constantly write to inquire how 

 the effect is produced. 



I have previously indicated the method in these columns, and 

 now in response to further requests, describe the mauuer of 

 photographing those multi])le images. 



The following is a simple jirocess :— The subject that is to 

 appear in the facets of the cornea has to be made transparent, 

 in other words a reduced negative has to be taken, and a 

 transparent positive copied from it. A convenient size for this 

 will be J in.— i; in. 



This may now be placed in the stop-holder of any condenser 

 mount with, preferably, the condenser removed. All light 

 should bo blocked out around the transparency. 



Either i in. or i in. objectives may be us-ed, and the instru- 

 ment should be fitted to the photo- micro camera in the ordinary 



