February 22, 191 2I 



NATURE 



549 



^contents of the tube a pressure equal to the osmotic 

 pressure of the sugar solution, he finds that the ether takes 

 up from the solution the same quantity of water that it 

 would contain if it were in contact with water at atmo- 

 spheric pressure. 



I think it can readily be shown that these results can 

 only be due to coincidence. 



Consider the system in the figure, where a&, he, and hi 

 an: membranes permeable to water only, and the left-hand 

 j compartment is filled with a solution of 600 grams of 

 sugar to the litre — this is the solution that Prof. Trouton 

 luses — bearing in mind that (on the assumption that the 

 ! solubility of ether in the sugar solution may be neglected) 

 ithe surface of separation between the sugar solution and 

 the ether in the U-tube is a true semi-permeable mem- 

 tbrane, then on closing ah and he, and putting a pressure 

 I on the two pistons of p = 8o atmos. (the osmotic pressure 

 fof the sugar solution), we have exactly Prof. Trouton 's 

 "•rpr.riment. 



|%=o_ 



IN .ETHER 

 WATER I 



d 



■ The ether therefore takes up water from the solution 

 lUntil it is in osmotic equilibrium with it, but on opening 

 Hie membrane ah the solution is in equilibrium with the 

 water in the upper compartment, where the pressure is 

 <g = o; on opening the membrane be the aqueous ether will 

 also be in equilibrium with the water, for otherwise per- 

 petual motion would ensue : in other words, p is also the 

 osmotic pressure of the aqueous ether. 



Now, if I understand Prof. Trouton 's theory right, if 

 any other solution of sugar had been placed in the sugar 

 compartment under a pressure p', corresponding to the new 

 concentration, the ether would have taken up the same 

 quantity of water ; therefore, by a similar process of 

 reasoning as in the previous paragraph, the same solution 

 of water in ether has two different osmotic pressures — 

 which, of course, is impossible. 



A general method of determining the osmotic pressures 

 of substances not soluble in the liquid semi-permeable 

 membrane can be deduced ; it is, however, only a modifi- 

 cation of De Vries's original isotonic method. As already 

 pointed out, the ether takes up just such a quantity of 

 water as to give it the same osmotic pressure as the solu- 

 tion ; hence it is only necessary to obtain once and for all 

 a series of determinations of the osmotic pressures of vary- 

 ing concentrations of water in ether (these could be derived 

 by means of vapour-pressure measurements), and then place 

 the sugar solution and ether in the U-tube and determine 

 at what pressure the ether takes up enough water for it to 

 liave an osmotic pressure equal to the pressure actually 

 upon it. It must not be forgotten that in actual work a 

 correction will have to be applied for the varying solubility 

 of the ether in the sugar solution. Berkeley. 



Foxcombe, February 13. 



Microscope Stands. 



May I be permitted, as iiaving had a practical experience 

 in both English and Continental factories in the actual 

 manufacture and testing of microscopes, also in having 

 made a study of microscopists' requirements in general, to 

 pass a fpw comments on the subject of microscope con- 

 itruction lately dealt with in the letters appearing in your 

 (Columns ? 

 * In respect to the question of tripod versus horseshoe 

 ', certainly the tripod imparts stability to the instru- 

 it when in the horizontal position, but the same stability 



NO. 2208, VOL. 88] 



is also to be found in the horseshoe base as made by the 

 more modern makers, who have extended the rear toe, or 

 bearing point. The latest stands of a certain Continental 

 firm greatly excel in this latter feature, which permits of 

 inclination of the instrument to the horizontal with perfect 

 rigidity and safety. 



Moreover, the horseshoe base ensures portability to a 

 greater extent than the tripod ; and speaking from prac- 

 tical use, no matter which form of base is employed, in 

 photomicrographic work clamping is absolutely necessary 

 to ensure against vibration and the retention of perfect 

 alignment. 



The attachable mechanical stage, I have reason to 

 know, was originally brought out in order to provide 

 scientific workers with an accessory which could be utilised 

 in certain branches of work only, and which it is desirable 

 to be able to dispense with at will. 



The mechanical stage built into the microscope stand is 

 also made by Continental firms on a rotating principle, so 

 that it can be fixed in any desired position and can be 

 manipulated either from an angle or from the left or right 

 side of the instrument, the cross motion moving independ- 

 ently from the plate on which the object-slide rests in 

 order to avoid the fouling of the condenser, which is the 

 trouble experienced in mechanical stages built into the 

 stand and permitting of manipulation from one side only, 

 as is mostly the case in English-made stands of the pattern 

 referred to. 



In the case of curtailed motions, which are adopted in 

 order to avoid fouling, the motions become too short to be 

 of practical use in serial work. 



The use of mechanical stages is not advocated in certain 

 branches of scientific study, and is only required at times, 

 when the attachable form is most welcome. 



The necessity of a centring device to a substage con- 

 denser in preference to the fixed form is a matter for the 

 individual worker to determine. The question presents 

 itself as to how many students in microscopy know how to 

 centre a substage condenser. For all regular and ordinary 

 classes of work, other than the highest scientific investiga- 

 tions, a fixed condenser is the best and an advantage, 

 provided the objectives used are made to a strict standard 

 throughout and are fitted to the identical instrument on 

 which they are intended to be used. 



The advanced worker who finds it necessary to employ 

 a centring appliance would, from a practical point of view, 

 do well to select the centring device provided for attach- 

 ment to the objective and centre it to the condenser, this 

 form being more convenient to adjust in its position at the 

 end of the bodv-tube, than 'he centring screws fitted to the 

 substage undeVneath the stage, this latter arrangement 

 necessitating care being taken not to cut off the light 

 received off the mirror. 



If however, the centring substage is preferred, the same 

 is made by a Continental firm, and this also applies to 

 the objective centring device for use on ordinary micro- 

 scope stands. . , . 



Regarding the standardisation of substage fittings, it is 

 a mechanical impossibility to make smooth sliding fittjnfis 

 interchangeable where tubes are employed, one of which is 

 sprung in order to maintain a certain constant tension, 

 unless by resorting to a pressure screw, which arrange- 

 ment could, however, only be used when a ccntrin)? 

 appliance is employed. The standardisation of objectiv.-s 

 is a totally different matter, as greater latitude is pt^r- 

 missible in cutting the threads, the tension being obtained 

 at the shoulder of the objective mount, t.c. when the 

 objective is screwed right home. 



In respect to sprung slide fittings versus ground-in slide 

 bearings, it has been stated that the former arc more 

 easily adjusted by the microscopist. which dispenses with 

 the necessity of returning the instrument to the maker as 

 n the case of ground-in bearings; but it must be recog- 

 nise! that the constant adjustment necessary and ^-ver 

 at^dant when sprung fittings are employed in vort, ml 

 slides, i.e. coarse and fine adjustments and substage si,. .... 

 immediately throws these bearings-responsible for the 

 pToTer carrving of the above adjustmcn.s-and objectives 

 Snd condenser, out of alignment with P^^h other. 



The adoption of slide fittings requiring to be sprung is. 



