Iodine by Multiplex Excitation, 679 



graphing with the apparatus, the small gear-wheel is thrown 

 out of mesh with the large one by sliding a wooden support 

 through which the rod passes a little to one side, the object 

 being to prevent the transmission o£ vibrations from the 

 grating-house and the long wooden tube to the grating 

 support. The wooden tube, down which the light from the 

 slit passes to the lens and grating and back again to the 

 plate or eyepiece, was supported on posts and roofed over 

 to prevent it from being heated by the sun. The end of the 

 tube passes through the wall of the grating-house, but does 

 not come in contact with the lens support. The plate-holder 

 and slit were supported on a shorter piece of water-pipe, 

 which passed through the floor of the laboratory without 

 contact with it. The camera consisted of two wooden boxes, 

 one sliding within the other (for focussing), joined to the 

 end of the long tube with black cloth, which shut out the 

 light but did not transmit vibrations. The long tube was 

 made by nailing eight-inch boards together, and was painted 

 black on the inside. Some trouble was given by spiders, 

 which built their webs at intervals along the tube, a difficulty 

 which I surmounted by sending our pussy-cat through it, 

 and subsequently destroying the spiders with poisonous 

 fumes. 



The grating-house was built with double walls, and 

 shingled on the outside, the shingles being nailed to wooden 

 strips so as to leave an air-space between them and the roof. 

 The iron pier was shielded from the wind by a wooden box 

 built around it, which reached from the ground to the seven- 

 inch hole in the floor of the grating-house, through which 

 the pier passed without contact. 



I have mentioned these details of construction to show 

 that a somewhat elaborate apparatus can be put together out 

 of odds and ends, without going to the expense of castings 

 and machine-work. Photographs of the entire structure, 

 and the top of the pier and the grating and lens supports, 

 before they were housed in, are reproduced on Plate XJX. 



The apparatus has proved satisfactory in every way, the 

 only trouble resulting from air currents, which blow down 

 the tube and stir-up strise. These give no trouble at night, 

 and are practically absent on calm days, but on windy days 

 with hot sunshine they impair the definition somewhat. 

 The rest of the equipment which I installed for the work 

 consisted of a heliostat for work with sunlight, and a Cooper- 

 Hewitt mercury rectifier, which delivers 3*5 amperes at 

 220 volts for operating the mercury lamps. While this 

 spectroscope does not equal in resolving power the most 



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