Description of a Portable Diorama. 275 



geodetlcally, Tyith precision, the latitudes and longitudes of all the im- 

 portant points throughout the British Isles. 



May all these important labours be speedily brought to a satisfactory 

 conclusion^ for the benefit of both agriculture and commerce, since, in the 

 present state of our maps, the most palpable and dangerous errors, not- 

 withstanding all that has been urged for their correction, still continue 

 to exist, as ■will readily appear by an examination of the maps now sub- 

 mitted for inspection. 



*^* A few maps and charts were here exhibited, containing glaring 

 and dangerous errors to navigators. 



Description of a Portable Diorama, which may he viewed by a 

 number of persons at a time. By George Tait, Esquire, 

 Advocate, F.R.S.S.A. With a Plan. Communicated by 

 the Royal Scottish Society of Arts.* 



A portable diorama which I exhibited to the Royal Scottish 

 Society of Arts in November 184:1 and April 1842, and which 

 was honoured with their medal, could be viewed by only one 

 or two persons at a time, the pictures being within the box, 

 and being seen through eye-holes. t 



I have now made a diorama having the construction modi- 

 fied so that it may be viewed by a number of persons at a time, 

 the pictures being placed upon the front of a box, where they 

 are exposed uncovered. The front light is thrown upon them 

 from without, and the back light from within, the box ; and 

 both may be increased or diminished at pleasure. Gas is 

 the most convenient light ; but oil may be employed, by adopt- 

 ing means for properly increasing or diminishing the light 

 upon the pictures. The apparatus is used in a dark apart- 

 ment ; and ought to be so placed that the horizon of the pic- 

 tures may be on a level with the eye. The effect of coloured 

 sketches of a variety of changes which I made for the former 

 diorama, is equally satisfactory in this. 



The following side-elevation and plan represent a small 

 diorama made upon this principle : — 



* Head before the Royal Scottish Society of Arts, on 23d January 1843. 

 t Sec the printed Transactions of the Eoyal Scottish Society of Arts, and 

 the Edinburgh Now I'hilusopliical Journal for 1841, 1842. 



