388 DO AND DO NOT IN DRAWING ICe. X liable to cause confusion. If the mirror is silvered on the face or if a prism is used there will be no doubling of the reflected image. (8) Inverted images. One must follow carefully the directions or there is liable to be an inversion of the projected image (§ 512- 526). (9) In printing and photographing with projection apparatus the difficulties likely to be met with in photography are sure to come in. Knowledge of the principles underlying photographic pro¬ cesses will help one to overcome the troubles. § 549. Summary of Chapter Do i. Have a suitable room or a suitable shield around the draw¬ ing to keep out stray light (§ 453-455)- 2. Draw in the evening if a proper room is not available in the day time (§ 453). 3. Use an arc lamp for light if possible (§ 461-462, 486-487). 4. Always use a rheostat with an arc lamp, large or small (§ 487, fig. 182, 185). 5. One can draw images pro¬ jected by all forms of projection apparatus (§452). X: Do Not 1. Do not try to draw with the drawing surface flooded with stray light. Only the light from the specimen should reach the drawing surface. 2. Do not forget that it is dark in all rooms in the evening and, therefore, that is a good time to draw. 3. Do not use a weak light for drawing if you can have an arc light. 4. Never try to use an arc lamp, large or small, without a rheostat in series with it. 5. Do not forget that it is possible to draw the images projected by any form of appar¬ atus.