This is not a Dedication: such a Matter of Form I have ever thought beneath the Dignity of Philosophy. My Idea hereby is purely (after the Example of communicative Naturalists, proud of contributing to the Cabinets of the Curious any rare Fossil, or Article of the Study of Nature) to intreat that, in your immense Collection of Facts, Experiments and Researches, you will permit me to ask of you a small Nook for an Observation of a Phænomenon in the Animal Oeconomy: a singular Phænomenon, which has hitherto escaped the Attention of Physiologers, and which, I presume, is too curious not to excite your Attention. If, when the Occasion may present itself, any of the Members of your Society would be pleased to amuse themselves with verifying, by Dissections, the Elucidation of it, which is offered in the following Sheets, it might not, perhaps, be a regrettable Employ of Time. I am, with the most perfect Respect, _Gentlemen_ Your most humble And most obedient Servant, J. P. MARAT. Church-Street, Soho, January 1st, 1776. AN ENQUIRY, &c. Among the various Diseases affecting the EYES, there is one still[16] unknown, which Practitioners have hitherto confounded with the _Gutta Serena_. These are its characteristic Symptoms:--The Eye (when touched) becomes somewhat painful, without any apparent Cause; a Pression or Stiffness is felt inwardly; the lateral Motions of its Globe are performed with Difficulty; near situated Objects can no longer be seen; remote ones alone are distinguished at a fixed Distance, and even these imperfectly. This singular Affection of the Eye, which has yet no Name, but which may be termed _Accidental Presbytopia_[17] or Long-sightedness, is ever the fatal Consequence of taking prepared Mercury without proper Care. When Calomel, Panacea, Sublimate corrosive, or any other Mercurial Preparation in a saline Form, has been unseasonably administered, if not immediately evacuated by Purgatives, it passes with the Chyle into the Blood. As it circulates, it unites with the _Mucus_, with which it has a more intimate Affinity than with any other of our Humours. Afterwards it is carried into the glandular Organs, there to be[18] secreted. Whilst not yet united with the _Mucus_, if any of its Particles are of too large a Bulk to pass freely through the minute capillary Vessels, which it will have entered, it irritates their Coats with its sharp Angles: the Vessels contract; their Diameter being diminished thereby, the Lymph no longer permeates them freely; but, its Afflux continuing the same, distends their Cavity. The distended Vessels soon compress other minute adjacent ones; and these being obstructed and distended in their Turn, the whole Texture of the Organ they form is tumefied.[19]