"The light is always supposed to come from the upper left hand corner, at an angle of forty-five degrees.
"Imitations of wood or surface graining should not be attempted.
"The scale to which a drawing is made ought to be large enough to show the mechanism without crowding, and two or more sheets should be used if one does not give sufficient room to accomplish this end; but the number of sheets must never be increased unless it is absolutely necessary.
"Sometimes the invention, although constituting but a small part of a machine, has to be represented in connection with other and much larger parts. In a case of this kind, a general view on a small scale is recommended, with one or more of the invention itself on a much larger scale.
"Letters or figures may be used for reference, but they should be well made, and when at all possible should not be less than one eighth of an inch in height, that they may bear reduction to one twenty-fourth of an inch; or they may be much larger when there is sufficient space.
"Reference letters must be so placed in the close and complex parts of a drawing as not to interfere with a thorough understanding of the same, and to this end should rarely cross or mingle with the lines.
"The illustrations on pages of current topics under the head of new patents show the manner of putting in the reference lines from the letters to the part indicated.
"These are carried out some distance, but if placed on the face of the object where sectioned, a blank space should be left in the shading for the letter.
"If the same part of the invention appears in more than one view, it should always be represented by the same letter.
"Great care should be exercised in the matter of drawings, or they will be returned to the applicant, but, at his suggestion and cost, the officials will make the necessary corrections.