| 摘要 |
<p>The transparency supports each have a side notch so that they can be aligned. The notch is made by portable perforators or notchers which are manually operated and have an automatic actuating system which detects the presence of the item to be notched before operating. The notching may be done by knife, burning, chemical erosion, laser cutting etc. The notch is at a predetermined point on all slide transparencies so that they can be orientated rapidly before projection.</p> |
