For a blind or partially sighted person, a diagram may not be helpful if it is very complex or the whole thing cannot be seen or felt at once. An alternative is to ask for the diagram to be described verbally. If this is not practical in the teaching situation, consider spending some time with the lecturer before or after the session. It may be helpful to record your description on to tape so that you can refer to it later. A support worker or learning assistant may be able to do this for you but thought needs to be given to how to describe in a meaningful way.
Appropriate lighting can make a considerable difference for people using their residual vision.
Simply increasing the level of light may not be the answer for all students, so this needs to be discussed. A larger version of a diagram can be provided either by drawing an individual tailor made copy or by photo enlarging the original. Bear in mind that several large pages may be needed especially if the diagram is complex which may make it difficult to refer to and to handle. A disadvantage is that such aids allow access to only one part of a diagram at a time. The greater the degree of magnification, the more restricted is the field of vision and the harder it may become to understand the diagram, to the point where seeing the relationships of the parts may be impossible.