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MacKenzie was a Glaswegian by birth and education. His interests in ocular anatomy and physiology developed early, and were developed during his years in London and on the Continent. He returned to Glasgow where he was instrumental in founding an Eye Infirmary. His book A Practical Treatise on Diseases of the Eye, published in 1830, was widely adopted. He is shown in a diagram of the eye from the book, drawn by a colleague, Thomas Wharton Jones (1808-1891); a tonal version of the diagram was selected as the frontispiece. The book ranged widely and the following quotations are taken from it. The first concerns developmental changes in visual acuity: “Although the eye, after middle life, loses the power of distinguishing near objects with correctness, it generally retains the sight of those that are distant. Instances, however, are not wanting of persons of advanced age requiring the aid of convex glasses to enable them to see distant, as well as near objects.” He also wrote insightfully about strabismus: “In this disease, although the patient means to look at the same object with both eyes, one of them, moving involuntarily, and independently of the motions of the sound eye, turns away from its natural direction. If the sound eye be now closed, the other generally returns to the proper position, and so long as it is used alone, can be carried by the will of the patient in any direction he pleases. The instant, however, that the sound eye is again opened, the one affected with strabismus revolves inwards or outwards, and there it remains, not harmonizing in the movements of its fellow, or if it does move along with the sound eye, yet never so as to permit the two axes to be pointed at the same object. Hence the patient sees double, especially in the commencement of this disease; but after it has continued for a length of time, the double vision wears off, the impression on the squinting eye going for nothing. The eye is much more frequently distorted inwards than outwards in this disease, the adductor seeming to overpower the abductor, or the obliqui overcoming the recti. The former case is termed strabismus convergens, and the latter divergens. In some individuals we find the eyes squint alternately, or both together. In one case only have I seen strabismus directly upwards.” MacKenzie was highly regarded and was appointed Queen Victoria’s Oculist for Scotland.