238 



KNOWLEDGE. 



[October, 1901. 



I have recently been working with Ashe's Camera Lucida 

 with the modifications described by Mr. Scourfield in the 

 Jonrniil of the Quekett J[icrosco])icaI Club for 190it, and 

 believe that for many purposes this will be found the most 

 practical and convenient pattern of camera. 



It combines the ease of working of the Beale's neutral tint 

 without the transposition of the object and has not the dis- 

 advantage of bulk possessed by the Abbe Camera. 



It can be used at any angle to which the V)ody of the micro- 

 scope may be inclined, from 4.'i degrees to the horizontal, quite 

 comfortably, and by turning it round sidew.ays on the eyepiece, 

 it can be used at any angle from the vertical to 45 degrees. 



Its general construction can be gathered from the accom- 

 panying drawing. The image from the eye])iece is received 

 upon the mirror A, which consists of a silvered disc of micro- 

 scope cover glass mounted on a brass plate, which can be 

 revolved by the pin B, the image is then reflected to the 

 neutral tint glass C, which revolves on the pin D, and the same 

 effect is produced as in the Beale's pattern, excepting that there 

 is no reversal of the sides. 



I can strongly recommend the trial of this little device. 

 Much of the failure in drawing with camera lucidas is due to 

 the attempt to use eyepieces of too high power. It will be 

 invariably found that an eyepiece magnifying six diameters or 

 even less is the most satisfactory, and this equally applies to 

 Ashe's Camera Lucida. 



Colour Photo-Microgr.vphy. — It has often been deplored 

 that although very exquisite reproductions of delicate structure 

 can be made by photo- micrography, no satisfactory means have 

 been available for reproducing exquisite colour tints, which 

 make the vision of numerous objects through the microscope 

 80 entrancing. Attempts have been made, and not without a 

 marked degree of success, bj' means of the tricolour process of 

 Ives and others, but it called for a high degree of technical skill, 

 and a vast amount of patience, experiment, and time. 



The Sanger Shepherd process of natural colour photography 

 overcomes the majority of the difficulties which prevented 

 workers from embarking on attempts in this direction. It is 

 true that the results cannot be printed on paper, but must be 

 viewed as transparencies ; but they admit of ready exhibition 

 through a projection lantern, and for direct examination can be 

 held or supported towards a suitable white backing. 



The great advantage of it is that no alteration has to be made 

 to the ordinary camera. The recommended adaptation to the 

 camera consists of a repeating back to carry three plates. 

 Immediately in front of these plate-holders are fixed colour 

 screens, which are guaranteed to be of exactly the correct 

 absorption, and are adjusted by an improved form of Sir 

 AV. De W. Abney's colour .sensitometer. 



Three negatives of the same subject are taken, each with its 

 appro|)riate colour filter. One print is taken from each of these 

 negatives, and then stained by means of special solutions which 

 are supplied. The three prints are then bound together in 

 superposition to form a finished picture, and the result, if care 

 has been exercised, is very fine. 



Those who are in the habit of lecturing on microscopical 

 subjects, or who have hesitated to do so because they cannot 

 sufficiently reproduce the natural appearance of objects, should 

 make a trial of this process, and a very little practice with it 

 will cause them to be gratified with the results. 



NOTES AND QUERIES. 



C. J. R. Hkhards. — You will find the Platyscopic Lens a 

 very good one for the pur])ose you name. It is an Aplanatic 

 ^lagnifier, first introduced by J. Browning, of (jli, Strand, under 

 this name. There are many other good patterns, and in making 



a selection the principal points to consider are, the aplanatism, 

 the diameter of the visual field, and the working distance. Some 

 are now made in which these points have been specially studied. 

 Messrs. Baker, Beck and Watson all describe them in their 

 catalogues. 



F. K. E. — It would not be possible for a novice to judge of 

 the quality of objectives for himself. It requires experience 

 and a critical eye to decide on the comparative values. You 

 will be quite safe in trusting to any reputable firm for advice 

 in the selection of the object glasses. You would certainly 

 receive lenses of good performance from such. 



ir. I?. — To view multiplied images in the facets of the cornea 

 of a beetle's eye is quite simple. An easy method of doing it is 

 to place on the mirror a small cross cut out of black or brown 

 paper, about J" long ; illuminate in the usual way and focus 

 the facets with i" objective. Then gently rack the objective 

 upwards from the object, at the same time moving the paper 

 cross on the mirror, very slightly, with a needle point, and 

 the cross will appear in each of the facets. The needle 

 itself will probably indicate the direction in which the cross 

 should be moved in order to view it in the centre of the 

 facets. The real secret lies not in focussing the facets them- 

 selves sharply, but in racking the body upwards until the cross 

 comes into view, and focussing that sharply. 



Comnmnications and enquiries on Microscopical matters are 

 cordially invited, and should be addressed to M. I. CROSS, 

 Knowledge Office, 326, High Eolborn, W.C. 



NOTES ON COMETS AND METEORS. 



By W. F. Denning, f.r.a.s. 



The Gheat Southeex Comet (1901, I.). — The earth is now slowly 

 approaching this object again, but its distance is too great to encourage 

 the hope that it will be observed in anything but the largest instru- 

 ments. In Asf. Nach., 373-t, H. Thiele, of Bamberg, gives an 

 ephemci'is, of which the following is an extract, for Berlin niidnight : — 



Distance of 

 Date. E.A. Dec. Comet in Mil- 



H. M.S. a I liong of Jiiles. 



Oct. 3 ... 8 52 7 + 10 47 ... 322 



7 ... 8 52 45 10 4« ... 321 



„ 11 ... 8 53 7 10 50 ... 319 



Early in October the comet will be close to the stars a' and a" Cancri, 

 and with a very slight motion to the N.E. 



Encke's Comet. — This object lias been visible in the morning hours 

 since its discovery on August 5tli, but is now too near the sun to be 

 observed. Its rapid motion to the S.E. has practically carried it out 

 of the reach of obserrers in the northern hemisphere. 



AuorsT Meteors. — In the early part of August, 1901, moonUght 

 and cloudy weather prevented observation. The night of the 10th 

 wa'', however, very clear throughout at Bristol, and between 9ih. and 

 lob. the writer observed 102 meteors, of which 55 were Perseids, 

 from a radiant point at 44° + 58°. A considerable niunber must, 

 however, have eluded notice while the observer was engaged in 

 registering the apparent paths of those seen. Had the sky been 

 closely and uninteiTuptedly watched during the night, it is probable 

 that a single observer might have counted about 180 meteors. The 

 Perseids were not very numerous though they furnished more than 

 half of the aggregate number of meteors seen. 



The majority of the Perseids were small, but there was a fair 

 proportion of large ones, three being as bright as Jiqiiter, but no fire- 

 balls were seen. On August lltb, after a very stormy day, the sky 

 cleared in the evening, and until nearly midnight tlie conditions were 

 1 ery fa^ ourable at Bristol tor watching the shower. Between about 

 9h. 30m. and 12h. 30m. 72 meteors were seen, of which 49 were 

 Perseids from a radiant at 45° + 58 ". The Perseids were much more 

 frequent than on the precetUng night, and they were more brilliant, 

 l)ut the display was not a very rich one. The two brightest meteors 

 recorded during the night were as follow: — August 11th, lib. 2m. ; 

 magnitude twice as bright as Veiuis; path, 353^° -^ 7" to 3t3° - 14°. 

 The meteor left a streak for nearly a minute between the stars J and 

 '^ Aquarii, and its appearance was very similar to that of a bright 

 comet. August lltb, llli. otSm. ; magnitude equal to Venus; patli, 

 120' + 74 to 160 + 65 . A very intense streak was left a few 

 degrees N.W. of a Ursse lliijoris, but it died away in about 15 seconds. 



Tliougb the shower did not form a display of first-class import- 

 ance it seems to have been richer tlian usual, and to Iiave 

 exhibited activity until August 21st and probably later. The visible 

 character of an event of this kind is, however, always greatly enhanced 



