Many years ago, when this beautiful and useful optical instrument; was new and very little known, I wrote a paper in a scientific journal calling attention to its merits, and recommending its use. It was first described by its inventor in Jameson's Journal for 1828, p. 83. The title of the paper being “On a Method of so far increasing the Divergency of the two Rays in Calcareous Spar that only one Image may be seen at a time.” This paper was reviewed in Poggendorff's Annalen for 1833, p. 182, who says—That he perused Mr Nicol's account of his invention with very little hope of its proving successful, but that having constructed the instrument, he found that nothing could answer more perfectly than it did. Having read this testimony to its merits, I had one made by a London optician; which proved very successful.