BAIRD &. TATLOCK (LONDON) LTD. 



DESCRIPTION OF POLARISCOPES Nos. 5817-5825. 



This instrument is known as the half-shadow apparatus of Schmidt & Haensch. 



The tube v contains the illuminating system of lenses and is placed next to the lamp ; the polarising prism is at P (known as 

 a Jellet-Cornu prism, and which produces the half-shadow effect) and the analysing prism is contained in G. The quartz wedge 

 compensating system is also contained in G, and is controlled by the milled head x. The tube F carries a small telescope, through 

 which the field of the instrument is viewed, and just above is the reading tube L, provided with a mirror and magnifying lens 

 for reading the scale. 



The tube containing the solution under examination is shown in the trough between the two ends of the instrument. In 

 fusing this polariscope the lamp (ordinary white light) is placed at a distance of at least 20 centimetres from the end ; the observer 

 is so seated that the eye is in line with the tube F. The telescope is properly adjusted when the field of vision appears as a round 

 luminous disc divided into two halves by a vertical line passing through the centre. If the milled head T be rotated the appear- 

 ance of the field changes, one half becoming dark, the other half light, and finally a point will be found when the field is neutral , 

 or of the same intensity of light in both halves. 



The three different appearances presented by the field are shown in the following diagram : 



When the milled head is set at the point which gives the appearance of the middle disc as shown above, the eye of the observer 

 is raised to the reading tube L and the position of the scale with regard to the zero noted. It will be seen that the scale proper 

 is attached to the quartz wedge which is moved by the milled head : attached to the other quartz wedge is a small scale called 

 a vernier, which is fixed, and which serves for the exact determination of the movable scale with reference to it. On each side 

 of the zero line of the vernier a space is divided into ten equal parts. By this device the fractional part of a degree indicated 

 by the position of the zero line is ascertained in tenths : it is only necessary to count from zero, until a line is found which makes 

 a continuous line with one on the movable scale. With the neutral field, the zero of the scales and the vernier should coincide, 

 otherwise the zero point is out of adjustment. If, however, there is a deviation, a neutral field is secured at this point by turning 

 one way or the other the small key provided with the instrument, and which fits into a nipple on the left-hand side of F, the fixed 

 quartz wedge of the compensating system. Care must be taken not to confound this nipple with a similar one on the right hand 

 of the analyser, as the adjustment of the instrument would be seriously disturbed by moving it. Unless the deviation be greater 

 than 0.05 degree, it is enough to note the amount and make the necessary correction. 



The " setting " of the zero having been performed, the determination of the accurate adjustment of the instrument by means 

 of the control quartz plates is proceeded with. Three such plates should be used which have " sugar values " approximately 

 80, go, and 100 degrees respectively. These values may vary with the temperature, and tables are furnished with them which 

 give their exact value from 10 to 35 C. 



If one of these plates be placed in the instrument it will be noticed that the uniform appearance of the field is changed. The 

 milled head is turned to the right until the exact point of neutrality is re-established. The average from several such readings is 

 corrected for the deviation of the zero point, provided any was found, the deviation being added if it was to the left, and sub- 

 tracted if to the right. If the adjustment of the instrument be correct, the result should be the value of the control plate used as 

 ascertained from the table of the temperature at the time of observation. Each of the three plates is read in this way. 



The complete verification of the accurate adjustment of the polariscope as described above should be employed whenever it 

 is set for the first time, whenever it has sustained any serious shock or injury, and whenever it has been transported from one 

 place to another. It should also be done at least once a week when the instrument is in constant use. 



This instrument is adapted for use with white light illumination from oil or gas lamps. It is convenient and easy to read, 

 requiring no delicate discrimination of colours by the observer, and can be used by a person who is colour blind. This form of 

 instrument is adjusted to the Ventzke scale, which is defined to be such that I degree of the scale is the yJ T part of the 

 rotation produced in the plane of polarisation of white light in a column 200 millimetres long by a standard solution 

 of chemically pure cane sugar at 17.5 C. ; the standard solution of cane sugar in distilled water being such as to contain at 17.5 C. 

 in 100 cubic centimetres, 26.048 grammes of cane sugar. Ventzke degrees can be converted into angular degrees of D light by 

 multiplying by a factor which according to Rimbach for cane sugar is 0.344. F r instance, if a solution of a sugar read 45.6 

 Ventzke, then 45.6 X .344 = 15.46 angular degrees. This form of instrument only gives accurate results with solutions having the 

 same dispersive power as quartz : it is fairly accurate for the carbohydrates generally, but for substances such as terpenes, tartaric 

 acid, etc., the readings would be useless. 



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