[7] In this and future references to reflectors the Newtonian form is alluded to. The direct-vision reflectors of Gregory and Cassegrain have gone out of use, and the present popularity of Newtonians may be regarded as a case of the “survival of the fittest.” [8] Chambers’s ‘Descriptive Astronomy,’ 4th ed. vol. ii., also contains some useful references and diagrams. CHAPTER IV. _NOTES ON TELESCOPIC WORK._ Preparation.—Working-Lists.—Wind.—Vision.—Records.—Drawing.—Friendly Indulgences.—Open-Air Observing.—Method.—Perseverance.—Definition in Towns.—Photography.—Publications.—Past and Future.—Attractions of Telescopic Work. _Preparation._—An observer in commencing work in any department of astronomy will find it a very great assistance to his progress if he carefully reads and digests all that has been previously effected in the same line. He will see many of the chief difficulties and their remedies explained. He will further learn the best methods and be in the position of a man who has already gained considerable experience. If he enter upon a research of which he has acquired no foreknowledge he will be merely groping in the dark, and must encounter many obstacles which, though they may not effectually turn him from his purpose, will at least involve a considerable expenditure of time and labour. On the other hand, a person who relies upon guidance from prior experimentalists will probably make rapid headway. He will be fortified to meet contingencies and to avoid complications as they arise. He will be better enabled to discriminate as to the most eligible means and will confidently endeavour to push them to the furthest extent. By adopting existing instructions for his direction and familiarizing himself with the latest information from the best authorities he will in a great measure ensure his own success or at least bring it within measurable] distance. The want of this foreknowledge has often been the main cause of failure, and it has sometimes led to misconceptions and imaginary discoveries; for after much thought and labour a man will overcome an impediment or achieve an end in a way for which he claims credit, only to find that he has been anticipated years before and that had he consulted past records, his difficulties would have been avoided and he might have pressed much nearer the goal. Too much importance cannot be attached to the acquisition of foreknowledge of the character referred to, though we do not mean that former methods or results are to be implicitly trusted. Let every observer judge for himself to a certain extent and let him follow original plans whenever he regards them as feasible; let him test preceding results whenever he doubts their accuracy. We recommend past experiences as a guide, not as an infallible precept. It would be as much a mistake to follow the old groove with a sort of credulous infatuation as it would be to enter upon it in utter ignorance of theoretical knowledge. An observer