Were any person to attempt the construction of those telescopes, it is possible he might not succeed in his first attempts without more minute directions than I have yet given. The following directions may perhaps tend to guide the experimenter in adjusting the eye-tube to the speculum, which is a point that requires to be particularly attended to, and on which depends the accurate performance of the instrument. After having fixed the eye-piece nearly in the position it should occupy, and directed the instrument to a particular object, look along the arm of the telescope, from K (fig. 69.) to the extremity of the eye-piece at H, and observe, whether it nearly coincides with the object. If the object appear lower than this line of vision, the eye-piece must be lowered, and if higher, it must be raised, by means of the nuts and screws at _gd_ and _fe_, till the object and the line of vision now stated nearly coincide. The eye-piece should be directed as nearly perpendicular to the front of the speculum as possible, but so that the reflected image of one’s head from the mirror shall not interfere to obstruct the rays from the object. An object may be seen with an approximate degree of distinctness, but not accurately, unless this adjustment be pretty accurately made. The astronomical eye-pieces used for these telescopes are fitted with a brass cap which slides on the end next the eye, and is capable of being brought nearer to or farther from the first eye-glass. In the centre of this cap, next the eye, is a small hole, about the 1/40th or 1/50th of an inch diameter, or about as wide as to admit the point of a pin or a moderate-sized needle. The distance of this hole from the lens next the eye must be adjusted by trial, till the whole field of view appear distinct. A common astronomical eye-piece, without this addition, does not answer well. I find by experience, that terrestrial eye-pieces, such as those used in good achromatic telescopes, are, on the whole, best adapted to this construction of a reflecting telescope. I have sometimes used these instruments for the purpose of viewing perspective prints, which they exhibit in a beautiful and interesting manner. If a coloured perspective be placed at one end of a large room or gallery, and strongly illuminated either by the sun or by two candles, and one of the reflectors furnished with a _small magnifying power_, placed at the opposite end of the room--the representation of a street or a landscape will be seen in its true perspective, and will appear even more pleasant and interesting than when viewed through the common _optical diagonal machine_. If an inverting eye-piece be used--which is most eligible in this experiment--the print, of course, must be placed in an inverted position.