Sentence:  By an infinite number of materials, I mean colours and shades of all kinds and degrees; some notion of which variety may be formed by supposing a piece of white silk by several <WSD>dippings</WSD> gradually dyed to a black; and carrying it in like manner through the prime tints of yellow, red, and blue; and then again, by making the like progress through all the mixtures that are to be made of these three original colours. 
Meaning of the word marked with <WSD> tags:  An act or process of immersing.