The importance of microscopical observation as an adjunct to the study of disease is so very obvious that few can possibly deny its value. The great advancement that has been made in pathology, by the aid of the microscope, during the last few years, has almost completely changed most of the ideas that were formerly held as to the nature, prognosis, diagnosis, and treatment of many diseases. Diseases which were formerly shrouded in obscurity as to their real nature, are now clearly defined; we are enabled to trace the fons et origo of the causes which have been at work insidiously undermining the constitution, destroying gradually and surely important organs. We are enabled to employ a more certain means of diagnosis by discriminating structures from one another, which, to the unaided eye, seem alike—thus forming a more sure prognosis. Pathology has, in fact, become a new science by the aid of the microscope. And as microscopical observation becomes more complete, as better means of differentiating structures are discovered, while microscopic objectives are made of more perfect definition, so will the study of histology, and consequently pathology, rank higher and higher, till at last its place may almost, if not entirely, be raised to that of one of the exact sciences. The microscopist does not deal in hypothetical abstractions, but in demonstrable facts; truths which it is impossible to dispute, except from errors of observation. These errors are, however, in some cases formidable, for so many circumstances arise tending to obscure observation. Hence, there is a great necessity for co-operation in the work of histology and pathology, in order that these errors may be seen and understood, then grappled with and overcome. Observers must be willing to work more in unison, to compare results, to submit their work to the severest criticism possible: until they do so the science of histology will only slowly progress towards exactitude. More labourers, too, are needed; the field is so vast, time is so short, that the few earnest ones at present at work are not sufficient.