Jantarv. IQtJ 



KXOWLKDGE. 



37 



Tiombidoidea : family. Erythracidae ; genus, ICrythracus 

 (since altered by Horlcse to tirytliactinis. because the type 

 Hrytliraciis is a Kliyiicholopliusl. 



Kefereiices: — Hermann. J. F., " Menioire .Apterologitiue," 

 patre J7. Table 1. Fisjnre 12. Strasburg. 1804. 



Koch, K. L.. "Deuschlands Crustaceon," 1S35-41. 16. 2J. 



Herlese, F., 11, No. 4. Figure 2. 



The length of the mite is 0-96 millimetre. The figures to 

 ilhistrate mv note were Uindlv drawn hv Mr. Chas. D. Soar, 

 r.U.M.S. \ ^,■ V 



.\. \\ . .Alabaster. 



I).\KK GROUND ILLIMIN.-VTION .AND L"LTK.\- 

 MICROSCOPIC VISION.— The letter on the above subject 

 by Mr. F. Leitz is of considerable importance, for it is 

 eminently desirable that a proper understanding of the 

 province and limitation of the accessory appar- 

 .itus used in the production of dark ground 

 illmnination should be clearly defined. 



Particularly striking is the fact mentioned by 

 your corresspondent, th.it neither dark ground 

 illumination nor the so-called Ultra-Microscope 

 is a means of enhancing the resolving power 

 of the Microscope. Mr. Nelson pointed this out 

 some considerable time ago, and it has now 

 been ascertained that for an objective to utilize 

 its full resolving power the numerical aperture 

 of the Dark Ground Illuminator must be three 

 times that of the Objective. .As under present 

 conditions the maximum numerical aperture of 

 the Illuminators is 1-35. it follows that the 

 effective aperture of all Objectives exceeding 

 •45 is reduced. There is actually a greatci- 

 resolving power with dark ground illumination 

 with Objectives having numerical apertures in 

 excess of -45. but it is very substantially less 

 than that which is yielded with direct light ; 

 so much so that if we take 1 -35 as the limit of 

 numerical aperture for both condenser and objective, the 

 resolving power obtainable with dark ground illumination 

 is approximately th.at yielded with a numerical aperture of 

 •65 with ordinary direct illumination, while if the practical 

 limit of the objective be considered, namely 1-0 N.A., in 

 conjunction with a condenser of 1-35, the resolving power 

 obtained is only ecpial to that of an objective of -59 with 

 direct illumination. It therefore follows that the perception 

 of fine structural detail is not so nmch a feature with dark 

 ground illumination as the increased contrast afforded, and 

 the practical advantage of the use of high powers lies chiefly- 

 in the greater magnification. 



The well-known phenomenon of a streak of sunlight passing 

 through a small opening into a darkened room revealing dust 

 particles which in full daylight cannot be seen, is the 

 explanation of the effect produced in microscopical dark- 

 ground illumination when minute particles otherwise invisible 

 are revealed, and even with so simple a piece of apparatus as 

 the spot lens with a one-inch objective, starlike objects and 

 specks which by direct illumination would be invisible are 

 clearly seen and come under the category of "ultra- 

 microscopic." .And so through the whole range of dark 

 ground inunersion condensers, and finally the special 

 apparatus known as the " Ultra- Microscope " the same law 

 applies, only the method differing, and there is the same 

 limited resolution that is implied by the physical limitation of 

 the total numerical aperture of the condenser referred to 

 previously. 



Undoubtedly great confusion has arisen in the minds of 

 microscopists in consequence of the multiplication of dark 

 ground illuminators of various kinds, especially those which 

 have enabled objectives of high magnification to be employed, 

 and for the sake of clear understanding, and to avoid any 

 chance of confusion, it would be well to sharpl\' differentiate 

 the apparatus into three classes : — -, 



1. To comprise the Spot Lens and all the means until 

 recently provided for low power dark ground illumina- 

 tion, which might be classed as '' Low Power." 



F'IGURE 47 



2. .AH the modern Illuminators which have for their purpose 

 the use of high posvcr objectives and particularly the 

 revealing of living bacteria, and so on. which could be 

 classed as " High Power." 



.And linally, the entirely distinct arrangement which depends 

 on illumination at right angles to the optical axis 

 through a slit, which could be known as the " Ultra- 

 Microscope." 



I commend these brief notes to the consideration of your 

 readers, with the hope that someone more capable than myself 

 may pursue the matter further. .. . (-|,,>t-c 



.A Ni:W MICROSCOPE.— W. Watson 5; Sons, Ltd., have 

 recently introduced a Microscope of new design, the " Bactil 

 Mk. IV." which follows the lines of some well-known Conti- 

 nental models, but combines the special features 

 which distinguish ,all the Watson Microscopes. 

 Sec Figure 47. 



It will be noticed from the illustration that the 

 fine adjustment is set on the side of the limb, the 

 rotation of the controlling milled head being read 

 on a divided drum. This milled head actuates a 

 lever set in a vertical position. The freedom 

 from complications, certainty of action, and the 

 very slow movement that can be obtained is well 

 known in the lever form. 



The mechanical stage is not of the usual 

 .ittachable kind, but is built to the instrument 

 and gives a horizontal range of movement of two 

 inches. .A very neat feature of this stage is the 

 method of gripping the object slip. Hitherto this 

 has been dependent on a spring action pulling 

 ilown a curved finger-piece. This has alwavs 

 luiin open to the objection that the immersion 

 oil, especially if thick, exercised a retarding 

 mrtucnce on the slip, and this, pressing against 

 tlie spring referred to. caused it to give verv 

 slightly, with the result that when high powers were in use 

 there was an apparent loss of time on change of direction 

 of the mechanical screw of the stage. This is obviated in 

 this new pattern, the finger-piece being set by means of a 

 milled head, and being iunnovable excepting by its means. 



Hy an ingenious arrangement the whole of the stage can be 

 removed from the dove-tailed fittings and a plain stage 

 substituted for it if desired. 



The substage is of the regular English pattern with rack- 

 work to focus and screws to centre. It also lifts aside from 

 the optical axis. 



The sprung fittings to frictional surfaces, the large body 

 tube, the long range of coarse adjustment for use with low 

 power objectives, the advantage of which is so well-known, 

 are retained in this model. It is of solid and handsome 

 construction throughout. 



.APP.ARATUS TO FACILITATE THE ILLLMINA 

 TION OF OPAOUE OBJECTS WHEN VIEWED HV THi; 

 AID OF THE VERTICAL ILLUMIN.ATOR (Figure 48).— 

 This apparatus, suggested by .Mr. J. I!. Barnard, l-'.R.M.S.. 

 consists of a snuill right angle prism, fixed to an arm, which is 

 clamped to the nosepiece of the microscope by the screw of 

 the vertical illuminator, and has universal movements. It 

 will be found most useful when photographing opaque speci- 

 mens such as metals, and so on. In use the Microscope Stand 

 is placed in a horizontal position and the illuminating beam 

 thrown by the aid of a bullseye condenser upon the right 

 angle prism; it is then projected through the diaphragm of 

 the vertical illuminator, and so on through objective to 

 specimen; the diaphragm of vertical illuminator must of 

 course be directly under the right angle prism. By this 

 means of illumination the position of vertical illuminator may 

 be varied when objectives of different power are used without 

 the beam of light being altered, one of the inconveniences 

 experienced when the vertical illuminator is illuminated from 

 the side, and to overcome which many metallurgical micro- 

 scopes are provided with rack adjustment to the stage. In 



