436 



kn()\vli:dge. 



November, 1912. 



"Chambers' Hncyclopedia " (l')Oll, under Harvest HuR, 

 says it is Tronihiiliuiii holoscriccinn and Kivcs a figure of 

 tlie larva anil ailull. ll'ij;ures 4f>5 and 466.) 



It would be quite easy to give a large ninnber of references, 

 but those mentioned will show we are 

 some way from the correct solution yet. 

 We must keep on trying to breed the 

 larv.il forms from adults in confinement 

 of difl'ercnt species of mites until one 

 is found that t|uite agrees with Mr. 

 Marri.ige's figure and my own, 



Chas. D. Soak. 



DARK GROUND ILLLMINATION. 

 — Of late years, microscopists have shown 

 an increasing tendency to favour dark- 

 ground illumination. 



To this end, special condensers have 

 been provided, but chiefly for high-power 

 work. These have proved to be most 

 efficient in depth of black ground in their 

 defining powers, and also in their illumin- 

 ating properties. Perhaps it may not be 

 so well known that our ordinary con- 

 densers, the achromatics, and the Abbes, 

 are fully as capable of good work as 

 the new dark - ground condensers. 

 They can be specially eijuipped in 

 this direction for use 

 with low- .and medium 

 high power objectives. 

 .And the cost of this is 

 incon.siderable. 



The stand needs a 

 sub.stage fitted with a 

 focusing rack and centring 

 screws. The charm and 

 the comfort of dark ground 

 illumination is imdeni- 

 able, and when you can 

 add a beautiful effect and 

 perfect definition to these 

 advantages, you have an 

 irresistible combination 

 most desirable to possess. 

 The materials necess.iry 

 for this particular system 

 are four or more watch- 

 glasses, called "dead- 

 flats " by the watch- 

 makers. The thinner 

 kinds should be chosen 

 in order to provide room 

 for coloured screen discs 

 to rest in the same carrier. 

 They should fit the carrier 

 easily, yet without loose- 

 ness. Six Ihiti metal discs, 

 varying in diameter from 

 six-sixteenths of a inch 

 to eleven - sixteenths of 

 an inch, cut out with 



the lathe from sheaf-metal and bron/cd. 

 requirement. 



The nine-sixteenths of an inch disc may be taken to illustrate 

 the process by which the working result is attained. 



Spread any good cement, of the nature of seccotinc, lightly 

 over one surface of the metal disc, avoiding the actual edge. 

 Place the disc on or about the centre of the glass disc, and put 

 the latter, metal disc downwards, into the condenser carrier. 



Remove the upper lens of the condenser, and rack it up as 

 high as it will go imder the stage of the microscope. Fit a 

 two-inch power into the stand, and arrange the mirror so that 

 the fullest light may pass through the axis of the sub-condenser. 



Next, focus the two-inch objective upon the image of the 

 disc, to be seen through the sub-condenser with a ring of 



a Hra-Ming hy C. 11. 



Figure 467. 

 The Harvest Mite X 145 



Upper side of the Body. 



•^«c««»:^^'*rT 



FlGUUK 46S. The Harvest Mite X 145. 



Under side. 



the next 



white light around it. Then, with the centring screws, place 

 the image of the disc and its surrounding ring of light in the 

 centre of the field, and slowly close the iris diaphragm around 

 the image of the disc. If the disc is not centred with the iris 

 diaphragm, it ituist be delicately adjusted 

 with the finger tip, or other agent, until 

 the diaphr.igm sheaves close, and simul- 

 taneously shut out the light, all around 

 the circumference of the disc. 



When this is accurately done the 

 result can be tested with a familiar slide, 

 ■ . ; mounted for dark ground effects. First, 



>;• always see, by observing the image of 



the disc through the objective and con- 

 - "v den.ser, whether it is centred in the field. 



Then throw out the carrier and centre 

 ^ the fullest blaze of light you can get 



/ into the middle of the field by means of 



\.f the mirror, concave or plane. Replace 



the carrier and put your slide upon the 

 stage, and focus it with a two-inch 

 objective, or other power, up to one 

 inch, 

 I'v c. II. Soar. Now gently lower the substage con- 



denser upon its rack, and the light will 

 increase, the definition will grow, until 

 the full beauty of the subject of the 

 slide is apparent. For this experi- 

 ment, the iris diaphragm 

 must be fully opened and 

 the top lens of the sub- 

 condenser removed. 



Some condensers may 

 present problems in the 

 matter of focusing the 

 image of the disc and its 

 ring of light. Others, 

 again, have their carriers 

 so placed, that it is 

 difficult to reach the disc 

 underneath with the finger 

 for .idjustment. when the 

 iris is being closed around 

 the image of the disc. .\ 

 little careful ingenuity 

 will overcome all these 

 difficulties. .Always use 

 a very low power to 

 search for the image of 

 the disc. 



The proper disc for 



each objective must be 



found by experiment ; try 



e\ery diameter with each 



objective. The smaller 



the disc, the greater the 



light. Do not attempt to 



use any power higher 



ly C. n. ^oai. than one inch with a 



divided condenser. The 



discussion of methods 



that are used with powers 



exceeding one inch, and with the complete condenser, may be 



resumed in another paper. 



These methods differ considerably, and give the fullest 

 satisfaction when objectives not exceeding one-eighth of an 



inch are used. , i, \ n t- . 



(Rev.) R. Fr.ancis Joxes, 



DIOPSIS. — Insects which fly are, as a rule, provided with 

 two kinds of eyes, simple and compound, of which the former 

 are of use to them when at rest or when walking about, whilst 

 the latter give them distinct vision when in rapid flight. The 

 simple eyes in the case of flies are generally three in number, 

 and are set at different angles on an elevation upon the top of 

 the head, but the compound eyes occupy a large space at the 



