THE BOOK OF THE LANTERN. 269 



pearing suddenly on the sheet standing on its head, is an 

 episode which disturbs both lecturer and audience, and 

 will, for a time, entirely break the thread of the discourse. 

 This can easily be avoided if the slides be marked in a 

 certain way. If the slides are being used in fixed carriers, 

 and are therefore simply in the form in which they are sold 

 at the shops, each one should be marked with a white disc 

 at the lower left-hand corner of the picture, taking care 

 that the wafer is on the face of the picture, and by pre- 

 ference, beneath the cover glass, so that it cannot be rubbed 

 off. This white wafer will then come conveniently below the 

 operator's thumb when the slide is in its inverted position, 

 as it should be, before being placed in the lantern. 

 The operator will then become accustomed to look for this 

 white dot, which he can easily see even in a darkened 

 room, and he will place his thumb above it, and the picture 

 will of necessity appear on the sheet, as it should do. If 

 the back of the picture is placed next the light, of course 

 everything on the sheet appears in reversed order. What 

 should be the right hand of the picture appears on the left 

 of the sheet, and vice versa. This is of no great conse- 

 quence in some cases ; but if the picture should include 

 any lettering, such as that on a sign -post or a shop-front, 

 these letters will appear backwards, and the fault is at 

 once detected by the audience, and commented on by them 

 in audible whispers. I once had a volunteer assistant who, 

 at short notioe, supplied the place of my regular operator, 

 who happened to be ill. In the middle of my lecture this 

 man showed a slide upside down ; on seeing his mistake, 

 he took it out of the lantern and put it in again sideways. 



