548 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Septembkr, igo6. 



Conducted by F. Shillington Scales, b.a., f.r.m.s. 

 The Use of the Camera Lucida. 



'riii'Ki-; nrc siwral \va\s of ri'cordiiit; the appearance of 

 an object as seen in the microscope. Of these the 

 photo-microL;raph is consi(k'r<'(i the most trustworthy, 

 as it skives no \ariation with retfarci to the pow'ers of 

 oljservation and individual opinions of the oljserver, 

 powers of observation which vary with the oliserver's 

 skill and experience, and opinions w-hich too often 

 cn;il)lc him to persuade himself to see wiiat he wishes to 

 see, and which bias him according'ly by preformed hopes 

 or expectations. From the result of personal idiosyn- 

 crasies of this sort the photo-micrograph is free, and it 

 has even a peculiar property of its own in increasinsj 

 slitjhtlv the resolvins^ and, therefore, tlie dcfinin'^ 

 power of an ohjecti\e, but it has none the less great 

 disabilities. It takes no account of variations of focus, 

 and the higher the power and the greater the aperture 

 of an objective, the more is the ri'sulting photograph 

 limited to a sing-le plane, with all that this invobes. 

 It also takes no account, or even an apparently distorted 

 account, of differences in colour, and an object ^vhich 

 has been carefully differentiated by selective stains for 

 visual examination may, as a photo-micrograph, be con- 

 fused and misleading in spite of all the colour screens 

 and mono-chromatic methods of illimiination that ex- 

 perience can suggest. Therefore we are frec)uenllv 

 thrown l>ack on the pen and pencil as a means of re- 

 cording what we see for future reference or for the in- 

 formation of others. 



l-"or such pen and pencil methods the simplest plan of 

 all is to look down the microscope tube with one eve 

 and to record the result upon a piece of paper placed 

 conveniently at one's right hand. Hut the drawbacks 

 of such a method of procedure are manifest, and not 

 the least of these is the difficulty in keeping- the details 

 in place and in proportion. The methods used, there- 

 fore, come under two heads — the projection of the 

 image bodily upon a sheet of paper, and the tracings of 

 it there ;;/ si/n, or the use of an apparatus which enables 

 us with one and the same eye to see both the image and 

 the paper, and to draw each line of the former in its 

 proper position. 



The projection method requires the use of a pro- 

 jection microscope, an enclo.sed light, a darkened 

 room, and preferably a right angle prism to project the 

 imag^e down upon the paper as it lies upon the table, 

 thoug^h I have seen more or less unsatisfactory attempts 

 made to do this by means of a prism, such as the 

 Wollaston prism, attached merely to the eyepiece of 

 the microscope. It is fairly successful with low powers, 

 hut with high powers the loss of light is prohibitive, to 

 say nothing- of the other disadvantagfes inherent in this 

 method. 



The camera lucida as ordinarily used brings the 

 microscopical image and the paper into view at the 

 same time and in the same place. Of the many forms 

 of apparatus the simplest, the least expensive, and in 

 some hands not the least satisfactory, is Beale's well- 

 known form. .As originally designed by Amici this was 

 a piece of plain glass set at an angfle of 450 across the 

 eye lens of the ocular, the tube of the microscope being 



placed in a horizontal position and the observer looking 

 down through the glass at the paper beneath. By this 

 means he saw the paper directlx', whilst the microscopic 

 image was reflected up into his eye. There was, how- 

 ever, a troublesome and, indeed, fatal double rellection 

 from the inner surface of the glass, and Dr. Bcale got 

 over this difficulty by the simple expedient of using 

 tinted glass. Such a camera lucida can now he bought 

 for a few .shillings, or can even be home-made by anyone 

 possessed of a little ingenuity. Its main defect is that 

 the image is inverted though not transposed, and this 

 renders the subsequent filling in of details more trouble- 

 some and more liable to error and confusion. 



Wollaston's camera lucida is a form that wa.s early 

 introduced, and in many hands has also done g-ood 

 work, but is now more or less obsolete. It consisted 

 of a prisii-i covering- the whole of the eye-lens of the 

 ocular, and this prism u-as so constructed that it gave 

 two reflections and so transmitted the image to the eye 

 neither inverted nor transposed. The microscope was 

 used in the horizontal position, and the eye was so ad- 

 justed that half of the pupil looked into the prism and 

 the other half down upon the paper. .Any variation in 

 the position of the eve, therefore, caused the two fields 

 of view to become unec|ually illuminated, and the tlilfi- 

 cultv of keeping- the eye rigidly in the necessary position 

 caused this form of camera to fall into disu.se as other 

 and better forms were evolved. 



The camera lucida most used nowadays is that de- 

 signed or adapted by .'\l>be. It consists in its simplest 

 form of a ring to clamp around the eye-piece end of the 

 micro.scope, this ring carrying a projecting arm to 

 which is attached a mirror set at an angle of 45" or so 

 with the axis of the microscope. This niirr<ir reflects 

 the image of the paper into a small silvered mirror 

 placed above the ocular, thus reflecting the paper 

 into the eye. This latter mirror is perforated by a 

 ;:mall circular hole which enables the eye at the same 

 time to look directly down the microscope tube; in fact, 

 the eve is really looking through a cube of g-|ass cut 

 diagonally and with one of the diag-oiial surf.'ices 

 silvered and pierced as just described. 



The same principle is adopted in the comhin^'d camera 

 lucida and eye-piece sold by most opticians, which, 

 though less effective, are certainly more compact, a 

 mirror much reduced in size or a prism taking the place 

 of the larg-e Abbe mirror. 



In all forms of cslmcra lucida, other than those of the 

 direct projection type, the great difficulty is the adjust- 

 ment of the illumination so that neither microscopic 

 image nor paper overpower each other in brightness, 

 the former varying- g-reatly according to illuminant, 

 condenser, diaphragm, and magnification, whether of 

 objective or ocular. To avoid this, Abbe's camera 

 lucida in its most elaborate, and, unfortunately, its 

 most costly form, has a system of differently tinted 

 g-lasses, one set of which is used to adjust the bright- 

 ness of the imag-e from the paper, and the other set to 

 adjust the brig-htness of the microscopic image. There 

 is also a change of prism above the ocular \\ith a 

 different aperture for use with different objectives, ;md 

 even an elaborate centring mechanism. 



The use of the camera lucida always requires a cer- 

 tain amount of practice, but the initial diflnculties will 

 be rapidly overcome if the necessity for careful adjust- 

 ment of the two illuminations is borne in mind. The 

 differently tinted glasses are of g-reat service, and much 

 can be done by careful adjustment of the condenser 

 diaphragm, but, perhaps, the most useful hint that can 

 be given to the beginner is to suggest the use of two 

 lamps — one to illuminate the paper and one to illuminate 



