SCIENCE- G OS SI P. 



29 



MICROSCOPY FOR BEGINNERS. 



By F. Shillington Scales, F.B.JI.S. 



{Continued from Vol. VI. page 375.) 



Objectives should be carefully treated, and it 

 should be borne in mind that they are delicate 

 pieces of apparatus. Dew on a lens should be 

 allowed to evaporate ; dust on the back lens should 

 be removed with a soft camel-hair or sable brush ; 

 and if the lenses really require cleaning, a specially 

 soft piece of chamois-leather or cambric should be 

 kept free from dust and used for that purpose only. 

 The lenses should never be unscrewed : that is a 

 matter for a first-rate optician only, and the maker 

 is the proper man. An oil-immersion lens should 

 be carefully and gently wiped immediately after 

 use, and if by any chance any of the oil should 

 have dried on the lens it is best to put another 

 chop of oil on it, and to leave it for a time before 

 wiping clean. Under any circumstances use as 

 little pressure and friction in cleaning as possible. 

 When objectives are in use, but temporarily re- 

 moved from the microscope, they should be laid 

 end upwards on the table to keep out the dust. 

 For this reason it is well also to keep one of the 

 eyepieces habitually in the tube of the microscope. 



A glass shade is preferable to the ordinary 

 wooden case, except, of course, for travelling, as 

 the microscope is apt to get jarred or knocked 

 about through being constantly taken out of and 

 put into its case. It is well, however, to remember 

 that a microscope should not be allowed to stand 

 in direct sunlight, if for no other reason than that 

 the heat might prove injurious to the balsam or 

 cement connecting the lenses of the objectives. 



On p. 249 of the last volume of Science-Gossip 

 we gave instructions in the use of Beale's camera 

 lucida, and .what we said then applies very largely 

 to the other forms of camerae lucidae. The type 

 known as " Wollaston's " is not now much used, as 

 it has been superseded by other forms more easy 

 to use. It consists of a small prism placed over 

 the eyepiece, which reflects the microscopic image 

 into the eye. The microscope is inclined horizon- 

 tally, and the eye must be so placed that one-half 

 of the pupil is covered by this prism, and the other 

 half looks directly at the paper placed beneath. 

 The difficulty is in keeping this position, as any 

 ' displacement causes an unequal illumination of 

 either the image or the paper. Beale's camera 

 lucida has not this objection, and is, perhaps, at 

 present the most popular. A makeshift camera on 

 this principle can be made by means of a piece of 

 cork, a cover-glass, and a couple of pins ; all that 

 is necessary being to adjust the cover-glass in 

 front of, but at an angle of 45° with, the eye-lens 

 of the eyepiece. The cover-glass gives, however, 

 a somewhat troublesome double reflection ; and 

 this is obviated in Beale's arrangement, as supplied 

 by the opticians, by the use of tinted glass. The 

 Abbe form of camera lucida is considered by 

 many workers to be the best in the market, but it 

 is also the most costly. An arm projecting from 

 the camera carries a plane mirror, which reflects 

 the image of paper and pencil into a silvered 

 prism placed above the eyepiece, and so into the 

 eye. The paper must lie in the same plane as the 

 object ; and if the microscope is therefore to be 

 used in any but the vertical position, the drawing 

 board must be sloped accordingly. In both this 

 and the Wollaston form provision is made for 

 adjusting the light by means of tinted glass. Lately 



a new form of combined eyepiece and camera 

 lucida, made by Swift, Leitz, and other makers, has 

 found great favour on account of its simplicity. 

 It is made in two forms — one for use with the 

 microscope in a vertical position, and another in an 

 inclined position. A prism placed above the eye- 

 lens projects the pencil and paper clearly into the 

 field. Some little practice is also required with 

 this form to get the pupil of the eye placed in such 

 a position over the prism that neither the image of 

 the object nor of the paper overpowers the other. 

 The beginner will find that in all forms of camerae 

 lucidae the secret of success, as we have already 

 pointed out (see Science-Gossip, vol. vi., N.S., 

 p. 249), lies in the proper adjustment of the illu- 

 mination for both microscope and paper. It is 

 here that the value of an independent lamp for the 

 paper makes itself felt. With low powers the 

 illumination in the microscope is the stronger, and 

 the lamp-flame must be adjusted accordingly, or 

 even a piece of white paper may be placed over the 

 mirror when that is used. With high powers the 

 paper is generally the brighter, and tinted screens 

 must be used, or the light modified. The usual 

 standard for distance between eyepiece and table 

 is ten inches, and this should be adhered to ap- 

 proximately. Any variation will alter the size of 

 the drawing. It may not be superfluous to add 

 that short-sighted people will require to use their 

 spectacles if they are to see the paper and pencil 

 clearly. The pencil should have a sharp point, 

 and the lines should not be drawn too heavily in 

 the first place. With all forms of drawing apparatus 

 the paper must lie in the position for which the 

 camera lucida is designed, as detailed above, or the 

 result will be an elliptical image. 



The use of the stage-micrometer in connection 

 with the camera lucida will suggest itself to any- 

 one. It is only necessary to replace the object on 

 the stage by the micrometer, taking care not to 

 alter the other adjustments of the microscope, and 

 to note the measurements thus shown upon the 

 " drawing. Supposing the portion of the drawing to 

 be measured corresponded with one-hundredth of 

 an inch, as shown on the stage-micrometer, and 

 with one inch when measured with an ordinary 

 rule, the actual magnification is one hundred 

 diameters. If the micrometer be a millimetre 

 scale, it will be necessary to provide oneself with a 

 rule divided in millimetres — or to convert the 

 English measurements accordingly, either by re- 

 ference to a table or by calculation. For rough 

 purposes the English inch may be taken as 

 25'4 mm. 



The use of the stage-micrometer in conjunction 

 with the eyepiece micrometer has been dealt with 

 on p. 248 in the last volume. In making measure- 

 ments by this method when using high powers, 

 difficulty is often encountered in causing the object 

 on the stage or the stage-micrometer to come into 

 exact alignment with the lines in the eyepiece. To 

 obviate this a mechanical stage is a great con- 

 venience, or the form of micrometer designed by Mr. 

 Jackson, with a slight adjustment to the scale by 

 means of a screw. The most perfect form of 

 micrometer eyepiece is the screw-micrometer, 

 containing one fixed and one travelling wire ; the 

 movement of the latter being accurately recorded 

 by means of a drum, whilst each revolution of the 

 drum corresponds to one of many serrated teeth in 

 the field of view. 



(To be continued.) 



