BAIRD &TATLOCK (LONDON) LTD. 



Fig. I. 



5816 



5816' 



Polariscope, Schmidt & Haensch, Lippich's model half-shadow, with divided circle, 

 reading by magnification to 0.01, mounted on pillar with iron tripod base, with one 

 each tubes 100, 200, and 220 millimetres, with gas burner for sodium flame, in case 32 15 



INSTRUCTIONS FOR THE USE OF THE HALF-SHADOW POLARISCOPE, LIPPICH'S SYSTEM (Fig. 5816). 



In the above illustration p indicates the telescope, II the magnifying glasses, with protection cap and reflecting mirrors, K 

 the graduated dial, St the analysing Nicol prism, which is fixed to the revolving graduated dial and to the telescope, and h the 

 movable polariser, with the graduated segment A of a circle fixed to it, and B a little tube for bichromate solutions. The 

 apparatus can only be used with a sodium flame, and a suitable sodium lamp is supplied with it. The latter is placed at a 

 distance of 22 centimetres from the apparatus. It consists of a Bunsen burner (or a Barthel's spirit burner) supplied with a 

 platinum ring on which some pulverised sodium chloride is placed, and made intensely incandescent by means of the non- 

 luminous flame from the burner ; then the apparatus is pointed towards the brightest part of the yellow flame, which can easily 

 be accomplished by means of the adjuster provided with the lamp. 



The graduated dial, which is made to revolve by means of St with hand or by the micrometer screw m, after the 

 pinch k has been fixed, is as a rule graduated all the way round. In addition to whole degrees, half and quarter degrees are 

 indicated on the dial ; 24 such quarter degrees are divided on the two verniers into 25 divisions, therefore a scale mark on the 

 vernier coinciding with any one scale mark on the dial would indicate 0.01. Fig. I shows the inner revolving dial and the 

 exterior vernier ; the zero line of the vernier is now between the 13.50 and the 13.75 line of the dial ; the 0.16 of the vernier 

 coincides with some line on the dial, therefore the total reading is 13.50+0.16 = 13.66. 



If desired, a second scale can be provided on the dial to show directly grape or beet sugar percentages. This is done by 

 dividing the dial into whole percentages ; nine such are divided on the vernier into ten divisions, so that the vernier reads to one- 

 tenth per cent. The reading is done in the same manner as described above ; the beet sugar scale is based on the standard weight 

 of 26.048 grammes. The 100 line (100 per cent.) corresponds with a solution of 26.048 grammes of chemically pure sugar in a 

 100 cubic centimetre flask, examined with the 200 millimetre tube. The grape sugar scale is now rarely used, but if a tube of a 

 certain length (according to the most recent researches 189.4 millimetres) is used in connection with the degree scale, the grape 

 .sugar percentages can also be found directly, one degree in this case corresponding exactly to one per cent, of grape sugar. 



The Adjustment. When the above apparatus is well illuminated by the sodium flame, the zero position (the starting point 

 of all experiments) must first be found : this is indicated by the two halves of the field appearing equally illumined (equal half- 

 shadows). For this purpose the telescope F is focussed on the Lippich's polariser, so that the field presents a perfectly clear, 

 round circle divided into two equal parts by a sharply defined vertical line. If the graduated dial is turned through three or four 

 degrees to either the right or the left of the zero line, it will be seen that one-half of the field will become lighter, the other half darker. 



In the first place, the zero position is so adjusted that the zero line of the circle coincides with the zero line of the vernier. 

 The half-shadow can now be made lighter or darker (according as the polariser is turned to the right or left of the zero line) by 

 Iraeans of the pointer reaching from the dial-segment. When the pointer h is in the zero position, and at the same time the analyser 

 JA is placed in the zero position, both halves of the field of view appear black. The nearer the pointer is to the zero line, the darker 

 the half-shadow will become, and the more sensitive the apparatus ; but when the solutions are not quite transparent, the pointer 

 j-nust be moved more or less away from the zero line, so that the field is clear. For the majority of experiments the position of 

 j:he pointer at 7^ is most suitable, therefore the apparatus is usually so adjusted that in this position the dial and vernier read 

 : xactly o. When the pointer is moved, of course the zero point of the apparatus changes, and no longer corresponds with the 

 ,^ero line of the dial. The difference between the latter and the zero position of the apparatus must either be taken into account 

 I the simpler way) , or else after the graduated dial has been moved to o the apparatus must be again placed in the zero position ; 

 Ito do this, the analysing Nicol prism is turned, by means of the screws A, to the right or left until the half-shadows are equal in tint. 



Special attention must be called to the following circumstance, which, if not noted, may lead to considerable confusion. 



When the circle has been turned too far, and has gone beyond the sensitive range of the apparatus, the light, on comparison, 

 to a certain extent of the same intensity on either side of the vertical line, and this point maybe mistaken for the zero 

 i. Under these circumstances, even if the circle is turned through 10, 15, or even a greater number of degrees, hardly 

 my change will be observed. 



It is a matter, therefore, of the greatest importance, particularly after the sample to be examined has been placed in the 

 |ipparatus, to see that when the circle has been turned a few degrees on either side of the zero line, the transition from light to 

 hade, and vice versd, is instantaneous. 



On placing the sample to be tested within the apparatus, the first thing to do is to accurately adjust the telescope so that 

 he field is quite clear and equally divided by the vertical line, then the circle is turned until the shades are exactly of the same 

 ptensity on either side of the line. 



CHEMICAL AND SCIENTIFIC APPARATUS AND PURE CHEMICALS. 



105'J 



