The back of the front element of the objective with the prism in place

Although unsigned, this objective was likely made by Robert B. Tolles. It incorporates his built in system for vertical illumination consisting of a small prism located behind the front element of the objective.

The following was taken from 1890 edition of The Encyclopaedia Britannica.

Another method of vertical illumination long since devised by Mr. Tolles has recently been brought into notice by Professor W. A. Rogers of Boston (US). It consists in the introduction of a small rectangular prism at a short distance behind the front combination of the objective, so that parallel rays entering its vertical surface pass on between its parallel horizontal surfaces until they meet the inclined surface, by which they are reflected downwards. In passing through the front combination of the objective, they are deflected towards its axis; but, as their angle of convergence is less than the angle of divergence of the rays proceeding from the object the reflected rays will not meet in the focal point of the lens, but will be so distributed as to illuminate a sufficient area. By altering the extent to which the prism is pushed in, or by lifting or depressing its outer ends by means of a milled-head screw, the field of illumination can be regulated. The working of this prism with immersion objectives is stated by Mr. Tolles to be peculiarly satisfactory.

Here is an extract from the American Monthly Microscopical Journal, 1882.

A NEW FORM OF VERTICAL ILLUMINATOR.—A propos of the article we print this month, it may be well to once more call attention to the ingenious device of Prof. W.A. Rogers (we believe it was invented by him), made by Mr. Tolles. We refer to the reflecting prism fitted into the objective just back of the front lens. By throwing light upon the exposed face of the prism, it is reflected down through the front lens upon the object, and certainly gives a most excellent illumination. Prof. Rogers has used it in the examination of his ruled plates with great satisfaction. A member of the New York Microscopical Society, Mr. James Warnock, has recently received from Mr. Tolles, a 1/6-inch objective of large aperture, with this attachment, which gives a fine definition of the lines on Amphipleura pellucida. Before long we intend to allude to this subject again, and to give an account of a comparison between this and the common vertical illuminator.

Another example of a Tolles objective with a built-in vertical illuminator dated 1869 is located in the Harvard Collection.