16 ROYAL SOCIETY OF CANADA 



magnet. The probable error of a single reading was found to be about 

 -|- 0-06 sees., ,due to the above mentioned sources of error combined. 



19. The chronograph employed in the latest experiments although 

 of rude construction, had a smaller probable error than the stop watch 

 (viz: 1- 0.026 sees, for a single reading) and had the great advantage 



over the stop watch of enabling observations to be made contiuuausly, 

 without the necessity of waiting for an assistant to make each reading. 

 The chronograph consisted of a drum driven by a falling weight and kept 

 at nearly constant speed by clockwork over this drum (which was covered 

 with sand-paper) a long ribbon of paper (registering telegraph paper) 

 was made to pass by the rotation of the drum. In order to keep the 

 paper taut and to bring about the required friction between drum ana 

 paper, the ribbon was made to pass between the felt covered jaws of a. 

 spring clamp and a suitable weight was attached to the end of the 

 paper and descended, beside the driving weight, a distance of some 9 

 o]' 10 metres. The cord of the driving weight passed but once over 

 the drum and was held taut and prevented from slipping by a small 

 counter weight and the sand-paper; by this arrangement errors due to 

 inequalities of the diameter of the drum were avoided. A pencil 

 mounted above the paper where it was in contact with the drum and 

 operated by an electro-miaignet and key (placed at the observer's hand) 

 completed the apparatus. 



20. Besides the probable error, as given above, this chronograph was 

 subject to another error, due, probably, to variable friction of the clock- 

 work. The speed would very gradually increase or decrease and since 

 the amount was not calculable, as many readings were taken at a time as 

 possible and, where preliminary runs were not taken, and runs were be- 

 gun in the middle of a series, the results for all colours had to be dis- 

 carded in that series. 



21. The accidental errors, both with the stop watch and chrono- 

 graph, have been eliminated as far as possible, by large numbers of read- 

 ings. 



22. The apparatus employed to ascertain the effect of simple and 

 mixed coloured lights was a modification of Lambert's apparatus for mix- 

 ing pigment colours.^ Instead of pigments coloured glass plates were 

 used and placed over two square holes (cut in either end of a box). An 

 '' Auer " gaslight wias placed in the middle of the box iriside and reflec- 

 tors of white magnesium oxide (coating upon glass) placed inside the box 



^ Encyc. Britt., 9th Ed., Vol. VIII, p. 823. 



