src: They would be Waterhouse type, name of the guy who had the idea of making a slit in the tube to insert a plate with the hole in the center in order to cut peripheral light rays, you can see that opening in the photo above.
tgt: Eles seriam do tipo Waterhouse, nome do sujeito que teve a ideia de fazer um rasgo transversal no tubo para inserir uma chapa com o furo no centro e assim cortar os raios mais periféricos.