BAIRD &TATLOCKL (LONDON) LTD. 



DESCRIPTION. 



The calculating disc consists of a revolvable and a fixed set of graduations ; the outer fixed graduations are 

 situated on a flat ring of sheet metal, the inner on the periphery of a revolvable disc of the same material. Both 

 sets of graduations lie in the same plane, separated only by an extremely close-fitting space. They are placed 

 together, cased in cast metal, and protected from external influences by means of a round glass plate with a hole 

 bored in the middle. The axis of rotation at right angles to the disc is elongated upwards beyond the hole in 

 the middle of the glass plate, and carries a metal lever lined with cloth, which slides over, in contact with, the 

 glass, so that the disc can be rotated. The casing rests upon three feet and can be suspended upon the wall by 

 means of a loop. 



The ring graduations form together three scales. The first, headed " Refraktion," exhibits the scale divisions 

 60-34 f tne Dipping Refractometer divided into fifths. The second contains the specific gravities of 1.0080-1.0310, 



the intervals and the numbers 50, 60 300 indicate the units of the fourth decimal place, whilst " i.o " is 



omitted. The third scale exhibits like the first the scale divisions 34-60 of the Dipping Refractometer, but in 

 reverse order, and subdivided into tenths of a scale-division. 



The disc graduations have only two scales. The longer gives the extract, in agreement with the results of 

 the table of extracts by Windisch as grammes in 100 grammes beer from 4.0 to 8.6 per cent., one interval = 0.02 

 per cent., comformable to " Vereinbarungen zur einheitlichen Untersuchung und Beurteilung der Nahrungs- 

 und Genuchmittel fur das Deutsche Reich," vol. iii. (Arrangements for the uniform investigation and judging of 

 articles of food and drink for the German Empire) . The shorter scale indicates the percentage weight of alcohol 

 from 2 to 6 per cent., one interval =0.1 per cent.; the tenths of an interval, that is the hundredth percentage, 

 are to be estimated. 



30 25 210 15 110 BAIHD & TATLOCK. (LONOOM ) L 



'..EACH & LYTHGOE'S 

 REFRACTOMETER SCALE 



4992 



4992 T Leach & Lythgoe's Refractometer Scale 



Rd. No., 480847 



each 8 



DESCRIPTION. 



The Refractometer Scale is of use to every one who employs either the Abbe or ordinary butyro-refractometer 

 in the examination of fixed animal or vegetable oils and fats. In judging the purity of these substances, the 

 immense importance of the degree of refraction is becoming more and more apparent of late, so that to-day no 

 fat or oil analysis is complete without a refractometric examination, and every food or oil chemist must have in 

 his laboratory at least one form of refractometer. Some adopt the Abb6 instrument, which reads directly indices 

 of refraction, and some the simpler and less expensive butyro-refractometer, which, while originally intended 

 for the examination of butter and lard, has been found equally adapted to nearly all forms of animal and vegetable 

 oils. The latter instrument reads the refraction in degrees on an arbitrary scale, which may be transformed into 

 indices of refraction by computation or by means of a table. 



Thus it is that in analyses recorded by different chemists, refractive results are expressed in the two different 

 ways, some in butyro degrees, and some in indices of refraction. Again, from the fact that both the index of 

 refraction and the degree on the butyro instrument in the case of any instrument vary with the temperature, 

 and that it is not always convenient nor advisable to bring the temperature to exactly the same point in making 

 various observations, it follows that readings made and recorded at one temperature often have to be calculated to 

 other temperatures to be comparable with the work of another chemist, or even with one's own work at various 

 times. 



The Refractometer Scale is intended to serve the twofold purpose of readily transforming degrees of refraction 

 on the butyro instrument into the indices of refraction of the Abbe refractometer, and vice versa, and of showing 

 the equivalent of an observation made at one temperature to that made at any other temperature. 



The graduations at the extreme top and bottom of the scale express indices of refraction (D) from 1.4220 to 

 1.4900, while within and immediately contiguous to these are the graduations expressing degrees on the butyro- 

 refractometer. Thus each form of reading may readily be transformed in the other by a simple glance at the 

 scale. For example, a reading of 35.4 on the butyro-refractometer would correspond to an index of refraction 

 of 1.4491, and an index of refraction of 1.4762 would be equivalent to a butyro-reading of 76.3. 



The graduations on the sliding part of the scale at top and bottom express degrees of temperature centigrade 

 from 10 to 65. The inner graduations of the main rule contiguous to the slide at top and bottom are the butyro- 

 refractometer readings, arranged exactly opposite to the same readings above described, but further in on the 

 rule. The use of this part of the instrument may best be illustrated' by examples. 



Suppose the butyro-reading of an oil is recorded as 62.4 at 24. 5 C., and the operator wishes to compare this 

 reading with one of his own made at a temperature of 35. By moving the slide so that the temperature 24.5 is 

 opposite the reading 62.4 he will find that the equivalent reading at 35 would be 56.7. 



Again, suppose a fat at 44 C. reads 42 on the butyro-refractometer, required the reading at 30 ; placing 

 44 on the slide opposite 42 on the butyro-scale, we find that 30 projects beyond the graduations on the main 

 rule. In this case, note the temperature on the slide opposite the extreme right-hand butyro-reading of 45.2 above, 

 which would be 37.9. Then move the slide to the left till 37.9 temperature is opposite to the extreme left-hand 

 butyro-reading of 45.2 below, and we find the required reading opposite 30 to be 49.3. 



If it is desired to transform the index of refraction at one temperature into its equivalent at another, one 

 simply reads in from the index of refraction to the equivalent butyro-reading and from this to the temperature 

 scale. Thus, suppose the index of refraction is given at 1 .4443 at 55.5, required the equivalent index of refraction 

 at 30, 1.4443 JKois opposite 28.7 butyro-reading. Placing the movable slide with 55.5 opposite 28.7 butyro- 

 reading, we find that 30 is opposite 41.7 butyro-reading, which in turn we find coincides with i.4536o. 





CROSS STREET HATTON GARDETST, E-C 



902 



