COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EVES. 341 



which did not from its nature need to be otherwise was painted black 

 to prevent the reflection of light. 



The definite arrangement and relation of the parts of the apparatus 

 as viewed in vertical elevation may be seen in Figure 1, while Figure 2 

 shows the horizontal plan. Against the north wall of the room was 

 placed a large wood-top table (T) ; at the right of this a lamp (>Sw), 

 giving as nearly as practicable a point of light ; and at the left, at an 

 e(iual distance from its centre, the apparatus (Lg, Lg') designed to 

 furnish the larger illuminated area (g). A third lamp ( T) was placed 

 directly above the middle point of the line joining the other two 

 lights,^ a line which may be designated as the directive axis (a, Fig- 

 ure 2). 



The lamp giving the small light (Sni) consisted of a wooden box 

 (22 X 22 X 35 cm.) painted black, and having a Nernst filament ar- 

 ranged before a small opening in the side facing the table. By this 

 arrangement a definite and clear-cut luminous area was secured. Since 

 there was no reflecting surface immediately back of the filament, the 

 light given off" in that direction traversed the blackened box and was 

 absorbed by the distant walls, two of which were set at an angle to 

 each other, as shown in Figure 1, to prevent, as far as possible, reflec- 

 tion of light. 



The filament used on this lamp was the regular 110-volt "single 

 glower " Nernst filament, which is about 15 mm. long and has a diam- 

 eter of approximately 1 mm. 



The illuminating apparatus (Lg and Lg') furnishing the large light 

 was more complicated. The lamp proper (Lg) was similar to Sm, 

 except that two 220-volt " six glower " filaments were used on a circuit 

 of the strength indicated.*^ The light of these two filaments was many 

 times stronger than that of the small lamp. In front of the lamp Lg' 

 was a long box (Lg,) lined throughout with white glazed paper, ex- 

 cept the end nearer the table, which was closed with a plate of ground 

 glass (g). The end farthest from the table made an angle of 45° with 

 the sides of the box (see Figure 2). An opening (o) in the side of the 

 box near the end farthest from the table admitted the light from the 

 lamp Lg', which fell at an angle of 45° upon the white paper covering 



2 The word " lights " is liere used to designate the luminous areas which fur- 

 nished the light used in the experiments. In the case of the lamps Sm and I', 

 these were Xernst filaments used directly ; in the case of Z7 it was a large stjuare 

 of ground glass illuminated by light from behind. The lamp T'was not used in 

 the experiments described in the present paper. 



3 The 220-volt current was secured by using a transformer on the same (llOr 

 volt) circuit tiiat supplied the smaller lamp. 



