or thinness, is it not probable, that the effect of the activity of the retina may be to alter its thickness or thinness, so as better to adapt it to reflect or transmit the colours which stimulate it into action?  May not muscular fibres exist in the retina for this purpose, which may be less minute than the locomotive muscles of microscopic animals?  May not these muscular actions of the retina constitute the sensation of light and colours; and the voluntary repetitions of them, when the object is withdrawn, constitute our memory of them?  And lastly, may not the laws of the sensations of light, here investigated, be applicable to all our other senses, and much contribute to elucidate many phenomena of animal bodies both in their healthy and diseased state; and thus render this investigation well worthy the attention of the physician, the metaphysician, and the natural philosopher?

November 1, 1785.

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Dum, Liber! astra petis volitans trepidantibus alis,
Irruis immemori, parvula gutta, mari. 
Me quoque, me currente rota revolubilis aetas
Volverit in tenebras,—­i, Liber, ipse sequor.

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INDEX TO THE SECTIONS OF PART FIRST.

A.

Abortion from fear, xxxix. 6. 5.  Absorbent vessels, xxiii. 3. xxix. 1. ——­ regurgitate their fluids, xxix. 2. ——­ their valves, xxix. 2. ——­ communicate with vena portarum, xxvii. 2.  Absorption of solids, xxxiii. 3. 1. xxxvii. ——­ of fluids in anasarca, xxxv. 1. 3.  Accumulation of sensorial power, iv. 2. xii. 5. 2.  Activity of system too great, cure of, xii. 6. ——­ too small, cure of, xii. 7.  Age, old, xii. 3. 1. xxxvii. 4.  Ague-fit, xii. 7. 1. xxxii. 3. 4. xxxii. 9. ——­ how cured by bark, xii. 3. 4. ——­ periods, how occasioned, xii. 2. 3. xxxii. 3. 4.  Ague cakes, xxxii. 7. xxxii. 9.  Air, sense of fresh, xiv. 8. ——­ injures ulcers, xxviii. 2. ——­ injected into veins, xxxii. 5.  Alcohol deleterious, xxx. 3.  Alliterations, why agreeable, xxii. 2.  Aloes in lessened doses, xii. 3. 1.  American natives indolent, xxxi. 2. ——­ narrow shouldered, xxxi. 1.  Analogy intuitive, xvii. 3. 7.  Animals less liable to madness, xxxiii. 1. ——­ less liable to contagion, xxxiii. 1. ——­ how to teach, xxii. 3. 2. ——­ their similarity to each other, xxxix. 4. 8. ——­ their changes after nativity, xxxix. 4. 8. ——­ their changes before nativity, xxxix. 4. 8. ——­ less liable to contagious diseases, why, xxxiii. 1. 5. ——­ less liable to delirium and insanity, why, xxxiii. 1. 5. ——­ easier to preserve than to reproduce, xxxvii. ——­ food, distaste of, xxviii. 1. ——­ appetency, xxxix. 4. 7.  Antipathy, x. 2. 2.  Aphthae, xxviii.  Apoplexy, xxxiv. 1. 7. ——­ not from deficient irritation, xxxii. 2. 1.  Appetites, xi. 2. 2. xiv. 8.  Architecture, xxii. 2. xvi. 10.  Arts, fine, xxii. 2.  Asparagus, its smell in urine, xxix.  Association defined, ii. 2. 11. iv. 7. v. 2. ——­