Go back to the Alchemy room. Look close at all the 4 numbered panels with drawings on 4 posts. The numbers on a panel when added horizontally, vertically and diagonally produces just one number. Calculate the one sum for each post and relate it to the 4 drawings on the same post. 15 for earth, 65 for fire, 34 for air and 175 for water.

Look close at alchemy table. There are 5 measuring spoons - 50, 25, 10, 5 and 1. There are several flasks of colored powder - which ones to use? Select the bottles with the 4 signs of the planets seen over the doors upstairs. How much for which powder? 15 for earth or Jupiter or black powder, 65 for fire or Mars or red powder, 34 for air or Saturn or blue powder and 175 for water or Venus or green powder. Click on a measuring spoon, then click on a bottle and then click on the flask to place the measured powder in flask. Add all the required amount of the 4 powder and then use the matches on the burner at the bottom of the flask to get purple gas vial. Take the 18th century gold coin beside the flask.

There are hidden drawings and letters on the fresco. Move the lit candle on the fresco very slowly. When the start of the hidden drawings starts to be seen, move the candle very slowly to highlight all the parts of the hidden drawing. At a certain spot in that drawing, the sketchbook will appear at bottom right of the page showing that Gus drew that particular hidden drawing and should not appear anymore. If the sketchbook does not appear move the candle over again very, very slowly and sometimes hold it in place to get Gus to draw it. Patience! The yellow X in the picture below is approximately where the sketchbook shows up (refer to the picture in the walkthrough for this - it may help you).

When all the hidden drawings and letters have been sketched by Gus, the sketchbook will automatically appear on the screen.