202 Permanence of the Ridlierfurd Photographic Plates. 



Operation 9. 



First Series. 



Scale. 



mm. 



74 

 73 



Screw. 



5.0040 

 7.0102 



DifE. 



2.0062 



Second Series. 

 Scale. Screw. Dlff. 



74 

 73 



5.0035 

 7.0090 



2.0055 



The micrometer readings in the column headed " Screw " are 

 the means of two separate settings in every case except the first 

 and last in each column. The latter are the means of eight set- 

 tings. It can be shown that the precision of the whole result is 

 very greatly increased by giving an increased weight to the first 

 and last readings in this way. The numbers in the column 

 " Diff." are found by substraction of the successive numbers in 

 the preceding column. They are, therefore, the distances on 

 the screw corresponding to the various millimetre spaces on the 

 scale. It will be found that every millimetre space on the scale 

 has been compared with the following spaces on the screw : 



5-7, 7-9. 9-1 1, 11-13, 13-15- 



The results are collected in the following table, which gives the 

 numbers from the column " Diff"." above minus 2". These quanti- 

 ties are expressed in units of the fourth decimal place. 



First Series. 

 Screw Eevolutions. 



Second Series. 

 Screw Eevolutions. 



Scale. 5-7 7-9 9-1 1 11-13 13-15 Scale. 5-7 7-9 9-1 1 11-13 13-15 



78-77 

 77-76 

 76-75 

 75-74 

 74-73 



-F-45 +39 +45 

 +69 +45 +55 

 +65 +45 +80 

 o +15 +50 

 +62 +8r +67 



+20 

 +30 

 +65 

 +35 

 +49 



H- 4 

 +18 



+ 19 

 — 9 



+ 2 



78-77 +39 +37 

 77-76 +40 +40 



76-75 +54 +80 

 75-74 — 4 +35 

 74-73 +55 +72 



+35 

 +50 

 +65 



+45 

 +58 



+64 

 +35 

 +55 

 +40 



+34 



+ 9 

 — 20 



+35 

 — 5 

 + 5 



Means. +48 +45 +59 +40 + 7 Means. +37 +53 +51 +46 + 5 



It is evident that the quantities in the above table are the 

 amounts by which the several millimetre spaces of the scale ex- 

 ceed two revolutions of the screw at various parts of its length. 

 Consequentlj^, the means at the foot of the columns are the cor- 

 responding excesses of the mean millimetre space. In other 

 words, if we take the mean millimetre space as our standard of 

 length, these means are the amounts by which the various dis- 

 tances of two revolutions of the screw are in error. A summation 



