222 



KNOWLEDGE. 



June, 1913. 



chelicerae, though, unfortunately, not also of the 

 hypostome. On looking through the mounted 

 specimens of some fifty species in my cabinet 

 I found that the chelicerae closely resembled 

 those of a Tick sent to me from Brisbane as 

 having been found on a dog in the neighbour- 

 hood of the Filbert River in Queensland. How, 

 then, could it have been found in Devonshire ? 

 Further enquiries elicited the fact that the 

 child on whose dress these Ticks were found 

 had shortly before been playing on a fur rug 

 made from the skins of Australian Dingoes, and 

 there seemed no doubt that these Arachnids had 



been imported in this. D „ . 



K. 1 . iw. 



A PORTABLE MICROSCOPE.— It is an 

 axiomatic truth that the Continental form of 

 microscope, with its short tube, lends 

 itself to portability better than the 

 English one, with its long tube and 

 extended base. 



Messrs. Leitz, in 1899, introduced a 

 portable model, in which a closing V 

 was substituted for the horseshoe foot ; 

 but in 1901 further improvements were 

 made by adopting the well-known plan 

 of pivoting the stage, so that it could 

 be turned round into the plane of the 

 optic axis ; thus the whole microscope, 

 although of full size, could be folded 

 up into a compact block, which could 

 be packed in a small box (see 

 Figure 232). 



Their later microscope model with a 

 horizontal pinion fine-adjustment (in- 

 troduced in 1903) has now been 

 adapted to this portable form, and one 

 has just been made in which some 

 alterations have been adopted. 



First, the stage has been made on my 

 horseshoe plan, and fitted with a sliding 

 bar (Figure 231). It measures 13 X9cm. ; 

 rather a contrast to the 6X4 cm. stage, 

 at one time an extreme size for a 

 Continental microscope ! 



Attached below the stage is the 

 usual spiral focusing mechanism carrying 

 a sub-stage ring, which has a small amount 

 of play for centring by 

 means of a spring 

 opposing two screws. 

 A tube sliding into this 

 ring carries an achro- 

 matic condenser and an 

 iris diaphragm ; below 

 this is a turn-out ring, 

 with a rather deep cell, 

 to hold screens, stops, 

 and so on. 



Secondly, the pillar 

 which carries the 

 microscope is fitted 

 with a conical pivot, 

 so that the V-foot may 

 be turned round in 

 order that the toes of 

 the V may face back- 

 wards instead of for- 

 wards (Figure 231). 

 The rotating foot was 

 first made by John 

 Cuff, 1750. Previously, 

 when the microscope 

 was inclined it was 

 found that its stability Figure 232. 



was seriously impaired, The Microscope folded. 



Figure 230. 



Chelicerae of the 

 Rhipicephalus. 



Figure 231. 

 A Portable Microscope. 



so much so that when a horizontal position 

 was reached the microscope fell over ; but 

 now, when the foot is rotated backwards, the 

 microscope is perfectly stable. Figure 233 

 shows the instrument holding an Abbe camera 

 in a position for drawing at the side, an increase 

 of stability being required on account of the 

 weight of the large mirror on the top of the tube. 

 In this case the V-foot is rotated half-round. 

 The draw tube has been increased in length, so 

 that with eighteen millimetres for the objective 

 slide or rotating nosepiece, a total of two 

 hundred and eleven millimetres has been ob- 

 tained. There are some objects which require 

 the whole of that length, even with a short tube 

 objective. 



In Figure 231 the instrument is seen fitted 

 with one of Messrs. Leitz"s eyepieces, 

 with rotating eye-lenses (introduced 

 1899) ; the same field lens does duty for 

 both. This is a most convenient and 

 time-saving device. One is a No. 2 

 (X6), the other a No. 5 (X12). The 

 corrections of these eyepieces are very 

 perfect. 



The objectives, six in number, viz., 

 Nos. 1, 2, 3a, apo. 8 mm., 6a and apo. A 

 1-4 N.A., are all supplied with slides, 

 and fit, while attached to their slides, in 

 a separate box : this plan saves much 

 time in screwing and unscrewing and 

 packing them away in their own brass 

 boxes. This box is itself packed in the 

 same case as the microscope, as also 

 are the six brass boxes for the objectives, 

 and an oil bottle. 



In addition to the rotating eyepiece, 

 there are Nos. 2 and 3 Huyghenian, a 

 micrometer eyepiece, and an 8, a 12 and 

 an 18 compensating eyepieces. A polariser 

 and analyser has also been added, the 

 whole thus forming a fairly complete 

 microscopical outfit. 



Edward M. Nelson. 



PHOTOGRAPHY. 



By Edgar Senior. 



IMPERFECT FIX- 

 ING. — Hypo " Sodium 

 Thiosulphate " is in- 

 variably used as the 

 fixing agent for both 

 negatives and prints. 

 Its action depends upon 

 the formation of a 

 double salt with the 

 unaltered silver salt left 

 in the film, and when 

 the hypo is present in 

 sufficient excess the 

 double salt formed is 

 highly soluble in water 

 and diffuses readily out 

 of the film in washing. 

 If, however, the fixing 

 bath is too weak, or the 

 plates or prints are 

 removed too soon, then 

 imperfect fixing results, 

 owing to the formation 

 of a double salt which 

 is almost insoluble in 

 water. This may be 

 shown as a simple test- 

 tube experiment by 



Figure 233. The instrument with 



draw tube increased in length and 



fitted with an Abbe camera. 



