206 LECTURES TO SCIENCE TEACHERS. 



of lights. For large lights Fresnel employed ranges of 

 lenses below and above the central drum which parallelised 

 upwards and downwards the rays from the lamp which 

 were there reflected horizontally by corresponding parabo- 

 loidal mirrors. 



I must not omit to refer to Fresnel's first catadioptric 

 apparatus used for lighting the canal St. Martin, con- 

 structed in 1825, and to this wooden model for an- 

 other similar but larger one. There is also exhibited 

 here his first lens, polygonal in form, in consequence of 

 the difficulty at that time of executing lenses in glass. 

 Here are besides two other lenses, one polygonal, the other 

 circular, for lights of the first order (this one being in 100 

 pieces). 



Before proceeding further I shall here notice the various 

 improvements in reflector lamps. The portion of the para- 

 boloid behind the focus is replaced by a spherical mirror 

 which throws back the light to the flame to be transmitted 

 to the front. The upward and downward rays are caught 

 by a lens whose position is determined so as to catch the 

 rays from the lamp which would not meet the mirrors, 

 without interfering with the action of the reflector. The 

 lens can be placed either inside the reflector or altogether 

 outside, leaving its centre open to let the rays from the 

 reflector pass through. 



M. Bordier Maret invented a double reflector sending 

 light in two opposite directions, composed of two parabo- 

 loidal reflectors, back to back, with a common focus. If 

 this section is made to revolve round a vertical axis pass- 

 ing through the common focus, a lantern at this focus will 

 send its rays horizontally to all points of the compass. 

 A paraboloidal mirror revolving round the lamp gives 

 a flash at every point in azimuth in one revolution. 



I shall now describe the various improvements invented 

 and put in practice by Messrs. Stevenson. I shall first 

 take up the case of mirrors. Messrs. Stevenson employed 

 in 1814 the parabolic mirror with a lens in front to catch 

 the rays diverging beyond the reflector or mirror, and with 

 a spherical mirror behind the lamp, so as to turn back the 

 rays to the focus, and by these means he utilised almost all 

 the rays. Subsequently he invented a glass spherical mirror 

 of which Fig. 2 is a section. The diagram on page 208 



