SCIENCE- GOSS/J'. 



375 



MfCUOSCOI'V l()l< ItKGINNKKS. 



li. V. SiiiLLiNuroN S(ULS>, K.R.M.S. 



{CantittHfif /rfltn ptt^t 346.) 



UPAi^ih •ilijt.-ctsarvilhiiiiiiiuti.-d l>y scvunil iin.lho<ls. 

 The most l'rcc|>iviil Wiiy is to focus the lii;ht (lirci-lly 

 "(Xiu the iilijtct liy nu-.ins nf a hull's-cyc stand con- 

 (Icnsor, rcmcmlieriiit; thai the flat of the condenser 

 must be nearest to the object and (|uite close to it, 

 the focus beinj; short. If a lamp be used, it will be 

 necessary to raise it well above the stage. \ better 

 way is by the use of a •• side silver reflector," which 

 is a small, silver. [KiralKilic mirror, placed close to the 

 object and refleclmg the rays of light thereon from a 

 lamp placed nuitc near and about level with the 

 stage. Its management is soon learnt. Perhaps the 

 most perfect means of illuminating opaque objects is 

 by the now but little use.l •■ l.ieberkiihu." This is 

 really a speculum fitted above and round the ob- 

 jective, the light being thrown from bciifalh the 

 object and reflected down again upon it. It.s disad- 

 v.intages, and those which have caused it to be largely 

 disused, are that each objective must be fiUed with 

 its own IJeljerkiihn, and that the object inust be 

 mounted not upon a black liackground, but in such a 

 way as to give an annular ring of illumination round 

 it. When we deal with mounting, we will point out 

 that the generally reconnnended method of mounting 

 opa<iue objects upon a black background is not only 

 unnecessary, but inconvenient. The writer invariably 

 mounts such objects in an ordinary cell, and puts 

 under them a plain slide upon which a disc of black 

 (xiper has been fastened. What more is needed ? 



Before leaving the subject of illumination we will 

 deal with the buirseye condenser. In one of our 

 earlier papers (.SclENCE-Gossif, Vol. VI., N.S., 

 page 215) we have allude<l to the enormous spherical 

 and chromatic aberrations of this lens, and these 

 render it unsuitable for really critical work on account 

 of its bad definition. Kor this reason we do not 

 advocate its general use, save for opaque illumination. 

 .Should it be necessary, however, to fdl the field with 

 light by it, means (see page 2 1 5) we would ask our 

 readers to bear its optical properties in mind and to 

 remember that ti> obtain parallel light the conden.ser 

 must be placed close to the flame of the lamp, and 

 with its flat side against the flame. The bull's-eye 

 must, of course, also be placed both centrally and at 

 right-angles with the direction of the light. It is an 

 assistaiice to beginners if they do their focussing, 

 lioth with bull's-eye. and even with conden.ser, u|xm a 

 sheet of note-paper placed in the requisite position. 

 If the bull's-eye be used, it must be properly and 

 carefully adjusted, or- it will only interfere with the 

 proper focussing of the condenser. 



To pass now to the focu.ssing of the objective itself. 

 Thi.s needs but little explanation, but it may again be 

 advisable to point out (see page 157) that the .so- 

 called focal length of the objective does not in any 

 Hay represent its distance from the cover-glass of the 

 object. In fact, with increase of aperture, the 

 "bjcctives have got clo.ser and closer to the object. 

 When using high powers it is a help, and .sometimes 

 a preventi\e of damage, if the aperture of the stage, 

 as is customary in English stands, is made sufticientl)- 

 large to admit of the insertion of the linger under- 

 neath the slide so as to slightly lift or tilt its fore 

 edge. A high power can then be .safely brought 

 down upon the object, and, the appro.\imate distance 

 being found, the finger can be removed, and the 

 objective then brought gently to its ultimate focus. 



We do not think it advisable here to explain the 

 use of the correction -collar or adjustment of tube- 

 lenf;th for differelues in the thickness of cover-glass. 

 This has Ijeen fully dealt with in an earlier paper 

 (.S. iB.\CK-(J<)ssii', Vol.\T.,\..S.,p. 184); but it really 

 reijuires a trained .mil critical eye. as well as a critical 

 object, to enable these adjustments to Ik- sutisfacturilt 

 made. We may, however, remind our readers that 

 the oft-met-with instructions as to varying the magni- 

 fication by the simple device of alteiing the tulx.- 

 length are not really practicable save for very rough- 

 and-rea<ly work, and we recommend imr readers to 

 find out at the time of purchiising whether their 

 objectives are corrected for the 6i-inch or lo-inch 

 tube, and to reniemlicr that they "perform properly 

 only on the tube for which they are designed. Most 

 students' objectives (for an unsatisfactory reason con- 

 nected, we believe, with foreign competition) arc 

 corrected for the short tube ; but makers have a mis- 

 leading habit of giving in their catalogues complete 

 lists of magnificatiims calculated as if they were cor- 

 rected for the English lo-inch tube-length, .ind vouch- 

 safing no hint as to the real facts. 



There are a few more suggestions that may be use- 

 ful to the beginner. The first is to remember never 

 to use a stronger light than is necessary. Nothing is 

 more fatiguing to the eyes, or more likely to work 

 mischief, than exccs-sive glare ; but if reasonable pre- 

 cautions be taken, we do not believe the use of the 

 microscope is injurious to the eyesight. .Some people 

 are much troubled with what are called "floating 

 flies " in the eye, but this is to a certain e.\tent a 

 question of ease of position. The second rule is to 

 accustom oneself to keep both eyes open. The screw- 

 ingup of the eye not in use is a most injurious and 

 unncces.sary habit. .\t first, doubtless, some diffi- 

 culty will be found, and the eye that is not looking 

 down the tube will lie distracted by external influ- 

 ences : but this difficulty is only temporary, and a 

 little perseverance will overcome it. Some writers 

 recommend a shade, but we ha\e a<lvised many 

 beginners to commence simply by holding the hand 

 a short distance from the eye, and to gradually move 

 it further away as they gained experience, until finally 

 it was no longer necessary at all. All workers have 

 a tendency to use one eye more than another ; and in 

 this case the eye most used becomes generally rather 

 less sensitive to brightness of image, but more capable 

 of perceiving critical points. But every worker 

 should learn to use either eye with equal facility. 



\'arious tints of blue or yellow glasses, or a disc of 

 ground gla.ss, are useful for moderating the light, and 

 in some cases for accentuating the image. 



Remember to use no higher magnification than is 

 absolutely necessary. The real microscopist uses the 

 lowest power that will serve his pvirpose. for reasons 

 that a very slight acquaintance with the microscope 

 makes abundantly evident, and in all probability 

 the most generally used lens is that of tlie modest 

 inch. 



Remember also that the fine .adjustment is a deli- 

 cate piece of mechanism, and endeavour to .save it 

 (and the mechanical stage, if there be one) from the 

 very first. .\ny one of the microscopes we have 

 described will satisfactorily and easily focus a ^ inch 

 by means of the coarse adjustment alone. Do not 

 bear heavily upon any of the adjustments ; endea\ our 

 to balance them gently between the finger and thumb, 

 that the motion may be uniform : and do not oi\ any 

 account roll the fine .idjustment by pressing one finger 

 to one side imly of the milled head. 



( To he continued. ) 



